Cask and production method for cask

ABSTRACT

Heat conductive fins are welded along both sides of a belt-like member with a margin left along edges of the both sides, thereby to form a unit. The unit is welded to a body with the heat conductive fins from the outside of each fin. Next, another belt-like member is covered over a gap between adjacent belt-like members, and these members are welded from the outside. With this arrangement, there is no need to weld in a narrow and long space and the entire welding step is carried out from the outside, which makes it easier to assemble a cask.

TECHNICAL FIELD

[0001] The present invention relates to a cask that accommodates andstores spent fuel assemblies after finishing combustion and whoseassembly is easily performed, and a method of manufacturing the cask.

BACKGROUND ART

[0002] A nuclear fuel assembly that is at its end period of a nuclearfuel cycle, finishes its combustion, and cannot be used any more, iscalled spent nuclear fuel. The spent nuclear fuel containing highradioactive materials such as FP requires thermal cooling, and is,therefore, cooled at a cooling pit of a nuclear power plant for apredetermined period of time (one to three years). Then, the spentnuclear fuel is accommodated in a cask as a shielding container, and thecask is conveyed to a reprocessing facility by track or the like, and isstored there. Each spent fuel assembly is inserted into each cell of abasket installed in the cask, thereby to secure proper holding forceagainst vibration during the transportation.

[0003] There are various kinds of casks disclosed as conventionalexamples in “Nuclear Eye (in Japanese), Nikkan Kogyo Syuppan Production,issued on Apr. 1, 1998, and Japanese Patent Application Laid-open No.62-242725. A cask that has been helpful for developing the presentinvention will be explained below. The following contents will be shownonly for the sake of convenience of explanation, and do not mean knownor public contents.

[0004]FIG. 39 is a perspective view which shows one example of a cask.FIG. 40 is a cross-sectional view of the cask shown in FIG. 39 in itsaxial direction. The cask 500 is constructed of a cylindrical body 501,a resin 502 as a neutron shield provided on the external periphery ofthe body 501, external cylinder 503 thereof, a bottom section 504, and alid section 505. The body 501 and the bottom section 504 are made ofcarbon steel as a gamma-ray shield by forging. The lid section 505consists of a primary lid 506 and a secondary lid made of stainlesssteel or the like. The body 501 and the bottom section 504 are connectedtogether by butt welding. The primary lid 506 and the secondary lid 507are fixed to the body 501 with bolts made of stainless steel or thelike. A metallic O-Ring is provided between the lid section 505 and thebody 501, thereby to keep the internal side of the body airtight.

[0005] A plurality of heat conductive fins 508 are provided between thebody 501 and the external cylinder 503 to carry out thermal conduction.In order to increase thermal conductivity, copper is used as thematerial of the heat conductive fins 508. The resin 502 is injected intoa space formed between the heat conductive fins 508 in a fluid status,and is cooled and fixed. A basket 509 has a structure having 69 angularpipes 510 collected in a flux shape as shown in FIG. 26, and is insertedinto a cavity 511 of the body 501 so as to be restrained within thecavity 511.

[0006] The angular pipes 510 are made of aluminum alloy mixed with aneutron absorber (boron: B) to avoid the inserted spent fuel assembliesfrom reaching a critical state. At both sides of a body 512 of a cask,trunnions 513 are provided (one is omitted) to suspend the cask 500.Further, buffers 514 having wood built inside as a buffer material arefitted (one is omitted) at both ends of the cask body 512.

[0007] In the cask 500, the heat conductive fins 508 are welded to thebody 501, the external cylinder 503 is disposed around the fins, andwelding is carried out from the inside of the external cylinder.However, since the cask 500 has a height of about 6 m and the spaceformed between the external cylinder 503 and the heat conductive fins508 is extremely narrow and long, it is extremely difficult to manuallycarry out welding. Therefore, in the current status, an exclusiveself-run welding machine is manufactured, and the welding machine mustbe run in the space to carry out welding by remote control.Consequently, there has been a problem that the welding operation istroublesome. Particularly, when only a traditional welding machineexists in an assembling plant, it is difficult to assemble the cask. Asthe introduction of an exclusive welding machine and a technicalguidance become necessary, the assembling can be carried out at only aspecific assembling plant. This problem becomes serious in foreigncountries where the assembling technique has not been developed.

[0008] This invention has been achieved in order to solve the aboveproblem, and it is a first object of the invention to provide a caskwhose assembly is easily carried out and a method of manufacturing thecask.

[0009] Next problems of the conventional cask are as follows. FIG. 41 isa structural diagram which shows one example of the conventional cask.FIG. 42 is a cross-sectional view of the cask shown in FIG. 41 in itsaxial direction. In this cask 1500, a spent fuel assembly isaccommodated in a stainless body 1501 that shields the gamma ray (notshown) Around the external periphery of the body 1501, a plurality ofstripes 1502 are welded in a radial shape. These stripes 1502 have theircross sections bent in an approximately v shape, and have their sideedges 1521 welded to the body 1501, with the other side edges 1522welded to bent lines 1523 formed at corners of adjacent stripes 1502. Inorder to discharge decay heat conducted from the body 1501, the stripes1502 are constructed of a material having good thermal conductivity, forexample, copper plates or the like. A space formed between the stripes1502 is filled with a resin 1503 to absorb neutron.

[0010] A fin 1504 for heat radiation is welded to the outside of eachstripe 1502. This fin 1504 has a comb shape and has its tooth surfaceformed in a curvature. The decay heat of the spent fuel assemblies isfirst transmitted to the body 1501. As the stripes 1502 have been weldedto this body 1501, the decay heat is conducted from this body 1501 tothe stripes 1502, and is then discharged to the outside through theexternally exposed surface of the stripes 1502 and the fins 1504.

[0011] However, according to the above conventional cask 1500, as thestripes 1502 have the fins 1504 welded, there has been a problem thatthe temperature is distributed over the external surface, and it is notpossible to radiate heat uniformly. In other words, the temperaturedistribution occurs in such a manner that portions of junction with thefins 1504 have a high temperature, and the stripes 1502 between the fin1504 and the fin 1504 have a low temperature. This has not beendesirable in terms of heat radiation efficiency. This invention has beenmade in the light of the above aspect, and it is a second object of theinvention to provide a cask capable of improving the heat radiationefficiency.

DISCLOSURE OF THE INVENTION

[0012] In order to achieve the above objects, the cask of this inventioncomprises a body that accommodates a basket formed with a plurality ofcells where spent fuel assemblies are accommodated, with a plurality ofheat conductive fins provided around an external periphery of the body,an external cylinder fixed to an external periphery of the heatconductive fins, and a neutron absorber provided in a space formed withthe heat conductive fins and the external cylinder. The externalcylinder has a structure that a plurality of belt-like members A and Bare welded along edges of the members, the heat conductive fins arewelded along both sides of one belt-like member A with a margin leftalong edges of the both sides, and another belt-like member B adjacentto the belt-like member A is welded near the respective edges.

[0013] As the heat conductive fins are welded along both sides of thebelt-like member A with a margin left along edges of the both sides, itis possible to separate the welded joints between the belt-like member Aand the belt-like member B from the welded joints between the heatconductive fin and the belt-like member A, by welding the belt-likemembers A and B together near the edges thereof. With this arrangement,it is possible to prevent local concentration of thermally influencedsections.

[0014] The cask of this invention comprises a body that accommodates abasket formed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder. The external cylinder has a structure that belt-likemembers A and B are welded along edges of the members, a unit isstructured by welding the heat conductive fins along both sides of onebelt-like member A with a margin left along edges of the both sides, theunit is welded to the body with the heat conductive fins from theoutside of the unit and is arranged at predetermined intervals, andanother belt-like member B is covered over a gap between the belt-likemember A and the belt-like member A of adjacent units and is welded fromthe outside.

[0015] According to this cask, at the time of welding the belt-likemember with the heat conductive fins, and at the time of welding thebelt-like member to the body with the heat conductive fins in a unitstatus, it is possible to carry out the entire welding step from theoutside that is open, without welding in a narrow and long space.Further, as the heat conductive fins are welded along both sides of thebelt-like member A with a margin left along edges of the both sides, itis possible to separate the welded joints between the belt-like member Aand the belt-like member B from the welded joints between the heatconductive fin and the belt-like member A, by welding the belt-likemembers A and B together near the edges thereof. Therefore, it ispossible to prevent local concentration of thermally influencedsections. The cask includes the case in which the belt-like member A iswelded after the heat conductive fins are welded to the body, and thecase in which the heat conductive fins are first welded to the belt-likemember A and then the unit is welded to the body (hereinafter the same).Covering another belt-like member B includes the case in which thebelt-like member B is fitted in a gap between the belt-like members Aand A, and the case in which the edge of the belt-like member B issuperimposed on the edge of the belt-like member A.

[0016] The cask of this invention comprises a body that accommodates abasket formed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder. The external cylinder has a structure that belt-likemembers A and B are welded along edges of the members, a unit isstructured by welding the heat conductive fin along an approximatelycentral part of one belt-like member A, the unit is welded to the bodywith the heat conductive fins and is arranged at predeterminedintervals, and another belt-like member B is covered over a gap betweenthe belt-like member A and the belt-like member A of adjacent units andis welded from the outside of the member B.

[0017] At the time of welding the belt-like member with the heatconductive fins, and at the time of welding the belt-like member to thebody with the heat conductive fins in a unit status, it is possible tocarry out the entire welding step from the outside, without welding in anarrow and long space. Further, by welding the heat conductive fin alongan approximately central part of the belt-like member A, it is possibleto separate the welded joints between the belt-like member A and thebelt-like member B from the welded joints between the heat conductivefin and the belt-like member A. Therefore, it is possible to preventlocal concentration of thermally influenced sections.

[0018] The cask of this invention comprises a body that accommodates abasket formed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder. The external cylinder has a structure that aplurality of belt-like members A are welded along edges of the members,the heat conductive fin is welded along an approximately central part ofthe belt-like member A, the heat conductive fin is welded to the bodyfrom one side of the fin, and adjacent belt-like members A are weldednear the edges of the members.

[0019] At the time of welding the belt-like member with the heatconductive fins, and at the time of welding the belt-like member A tothe body with the heat conductive fins, it is possible to carry out theentire welding step from the outside that is open, without welding in anarrow and long space. Further, by welding the heat conductive fin alongan approximately central part of the belt-like member A, it is possibleto separate the welded joints between the belt-like members A from thewelded joints between the heat conductive fin and the belt-like memberA. Therefore, it is possible to prevent local concentration of thermallyinfluenced sections.

[0020] In the cask of this invention based on the above cask, thebelt-like members are formed in an angular shape or a valley shape bybending. By forming the belt-like members in the angular shape or thevalley shape by bending, the surface area of the external cylinderincreases. Therefore, heat radiation efficiency is improved.

[0021] In the cask of this invention based on the above cask, the heatconductive fin is fixed diagonally with respect to a radial direction ofthe cask. As the heat conductive fins are fixed diagonally with respectto the radial direction, it is possible to restrict the neutron from thespent fuel assemblies accommodated in the cask to pass through the heatconductive fins.

[0022] In the cask of this invention based on the above cask, the heatconductive fin is bent in an L shape or a U shape in its cross sections,and is in surface contact with the belt-like members.

[0023] The decay heat generated from the spent fuel assemblies withinthe cask is conducted from the body to the heat conductive fins, and isfinally radiated from the external cylinder to the outside. The heatconductive fins are usually made of steel having high thermalconductivity, and on the other hand, the external cylinder is made ofstainless or carbon steel in order to keep strength. Therefore, the heatconductive fins are bent in the L shape or the U shape in their crosssections, thereby to bring the heat conductive fins into surface contactwith the belt-like members, and secure thermal conduction from the heatconductive fins to the external cylinder. With this arrangement, it ispossible to improve the thermal conductivity of the cask.

[0024] The cask of this invention comprises a body that accommodates abasket formed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder. The external cylinder has a structure that abelt-like member A having an angular shape or a valley shape in itscross section is covered over external edges of adjacent heat conductivefins, and contacting edges of adjacent belt-like members A are welded.

[0025] At the time of welding the heat conductive fins to the belt-likemember A, and at the time of welding the belt-like member with the heatconductive fins, it is possible to carry out the entire welding stepfrom the outside that is open, without welding in a narrow and longspace.

[0026] In the cask of this invention based on the above cask, theadjacent belt-like members are coupled to each other along ends of themembers With this arrangement, it becomes easy to handle the belt-likemembers. Therefore, it is possible to assemble the cask easily.

[0027] The cask of this invention comprises a body that accommodates abasket formed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder. The external cylinder has a structure that aplurality of ring sheets are welded in an axial direction of thecylinder, a unit is structured by welding a ring-shaped heat conductivefin along an approximately central part of an internal surface of thering sheet, the unit is welded to the body with the heat conductive finfrom one side of the fin, and contacting edges of adjacent ring sheetsare welded from the outside of the ring sheet.

[0028] According to this cask, at the time of welding the ring sheetwith the heat conductive fins, and at the time of welding the ring sheetto the body with the heat conductive fins in a unit status, it ispossible to carry out the entire welding step from the outside that isopen, without welding in a narrow and long space. Further, by weldingthe heat conductive fin along an approximately central part of the ringsheet, it is possible to separate the welded joints between the ringsheets from the welded joints between the heat conductive fin and thering sheet. Therefore, it is possible to prevent local concentration ofthermally influenced sections.

[0029] The cask of this invention comprises a body that accommodates abasket formed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder. The external cylinder has a structure that ringsheets A and B are welded in an axial direction of the cylinder, a unitis structured by welding ring-shaped heat conductive fins to both sidesof one ring sheet A with a margin left along edges of the both sides,the unit is welded to the body with the heat conductive fins from theoutside of the unit and is arranged at predetermined intervals, andanother ring sheet B is disposed between the ring sheet A and the ringsheet A of adjacent units and is welded from the outside.

[0030] According to this cask, at the time of welding the ring sheetwith the heat conductive fins, and at the time of welding the ring sheetto the body with the heat conductive fins in a unit status, it ispossible to carry out the entire welding step from the outside that isopen, without welding in a narrow and long space. Further, as the heatconductive fins are welded to both sides of the ring sheet A with amargin left along edges of the both sides, it is possible to separatethe welded joints between the ring sheet A and the ring sheet B from thewelded joints between the heat conductive fin and the ring sheet A, bywelding the ring sheet A and the ring sheet B near the edges thereof.Therefore, it is possible to prevent local concentration of thermallyinfluenced sections.

[0031] In the cask of this invention based on the above cask, the unitand/or the ring sheet B is obtained by dividing the unit and/or thesheet into parts in a circumferential direction of the cask and weldingthese parts together.

[0032] By dividing the ring sheet in the circumferential direction, itis possible to assemble the ring sheets by fitting them from both sidesof the body, without the necessity of inserting the ring sheets from anaxial direction of the body. Therefore, it is possible to assemble thecask easily.

[0033] In the cask of this invention based on the above cask, theneutron absorber provided in the space formed with the heat conductivefins and the external cylinder is a molded resin formed along the shapeof the space.

[0034] In the above cask, the external cylinder has a split-typestructure and is assembled while sequentially welding divided parts.Particularly, in a case of welding the ring sheets by laminating them inan axial direction of the cask, the space between the external cylinderand the heat conductive fins is closed. Therefore, if the molded resinformed in the shape of the space is used, it is possible to dispose theresin while assembling the external cylinder.

[0035] In the cask of this invention based on the above cask, forwelding the heat conductive fins to the body made of steel, a bulgeportion made of iron is provided on the body, and the heat conductivefin made of copper is welded to the bulge portion.

[0036] In this cask, at the time of directly fixing heat conductive finsmade of copper to the body, the weldability of the fins is notsatisfactory. Therefore, the bulge portion made of iron is once providedon the body. Based on this presence, fine cracks that occur during thewelding are accommodated in the bulge portion, and the body can beprevented from being affected by the cracks.

[0037] The cask of this invention comprises a heat conductive fin and anexternal cylinder that are divided into a plurality of units. Each unitis connected around an external periphery of a body of the cask, andadjacent external cylinders are then connected together so as to enableheat conduction. Therefore, it is possible to assemble the cask easily.

[0038] The cask of this invention comprises an external cylinder formedwith a plurality of units that are disposed around an external peripheryof a body of the cask. Each of the units is integrally formed with aheat conductive fin and a wall of a part of the external cylinder inadvance, and the heat conductive fins, the body, and adjacent walls ofthe external cylinder are connected together to enable heat conduction.Therefore, it is possible to assemble the cask easily.

[0039] The cask of this invention comprises a body having a cavity thataccommodates fuel assemblies, a basket formed with sheet members thatpartition inside the cavity in a lattice shape, heat conductive platesprovided on end surfaces of the sheet member that are in contact withthe cavity, and a plurality of units each of which is integrally formedwith a heat conductive fin connected to an external periphery of thebody and an external cylinder covering over the external periphery ofthe body. Therefore, it is possible to assemble the cask easily.Further, the heat conductive plates can increase thermal conductivity.

[0040] The method of manufacturing a cask of this invention comprises astep of forming a unit by welding the heat conductive fins along bothsides of a belt-like member A with a margin left along edges of the bothsides, a step of welding a plurality of the units to a body of the caskwith the heat conductive fins from the outside of each of the units, anda step of fitting a belt-like member B into a gap between the belt-likemember A and an adjacent belt-like member A, and welding alongcontacting edges of the belt-like members A and B from the outside.

[0041] According to this method of manufacturing the cask, first, thebelt-like member A and the heat conductive fins are welded to provide aunit, thereby to facilitate the welding work. On the other hand, thebelt-like member A may be welded after the heat conductive fins arewelded to the body. However, in this case, the welding is carried outfrom the rear side of the belt-like member A, and therefore, the workbecomes slightly complicated (hereinafter the same). The latter methodmay be used for the cask.

[0042] For welding the units to the body, the welding is carried outfrom the outside of the units. At the time of welding the belt-likemember B by fitting it in a gap between the belt-like members A, thewelding is carried out from the outside. With this arrangement, it isnot necessary to carry out the welding using an exclusive weldingmachine in a narrow and long space like the conventional practice, andit is possible to carry out the entire welding step from the outside.Therefore, it is possible to assemble the cask easily. Particularly, ina foreign country where the assembling technique has not been developed,it becomes possible to assemble the cask using the existing ordinarywelding technique.

[0043] The method of manufacturing a cask of this invention comprises astep of forming a unit by welding a heat conductive fin along anapproximately central part of a belt-like member A, a step of welding atleast two units to a body of the cask with the heat conductive fin byarranging the units at predetermined intervals, and a step of coveringanother belt-like member B over a gap between the belt-like member A andthe belt-like member A of adjacent units and welding the belt-likemember B from the outside.

[0044] According to this method of manufacturing the cask, first, thebelt-like member A and the heat conductive fins are welded to provide aunit, thereby to facilitate the welding work. Next, the units are weldedto the body with the heat conductive fins. As the cross section of theunit has a T shape, both sides of the unit are open, and therefore, itis possible to carry out the welding work from the outside. Further, atthe time of welding the belt-like member B by covering it over a gapbetween the belt-like members A and A, the welding is carried out fromthe outside. With this arrangement, it is not necessary to carry out thewelding using an exclusive welding machine in a narrow and long spacelike the conventional practice, and it is possible to carry out theentire welding step from the outside. Therefore, it is possible toassemble the cask easily.

[0045] The method of manufacturing a cask of this invention comprises astep of welding a heat conductive fin along an approximately centralpart of a belt-like member A, a step of welding the belt-like member Ato a body of the cask with the heat conductive fin from an open side ofthe fin, and a step of welding a next belt-like member A to the bodywith a heat conductive fin from an open side of the fin, and weldingadjacent belt-like members A together along contacting edges of themembers A.

[0046] According to this method of manufacturing the cask, first, thebelt-like member A and the heat conductive fins are welded, thereby tofacilitate the work. Next, the belt-like members A are welded to thebody with the heat conductive fins. As the cross section of thebelt-like member A has a T shape, both sides of the fin are open, andtherefore, it is possible to carry out the welding work from theoutside. Further, at the time of welding the belt-like members A and Atogether, the welding is carried out from the outside. With thisarrangement, it is not necessary to carry out the welding using anexclusive welding machine in a narrow and long space like theconventional practice, and it is possible to carry out the entirewelding step from the outside. Therefore, it is possible to assemble thecask easily.

[0047] The method of manufacturing a cask of this invention comprises astep of forming a unit by welding a ring-shaped heat conductive finalong an approximately central part of the internal surface of a ringsheet A, a step of welding the unit to a body of the cask with the heatconductive fin in an axial direction of the cask from an open side ofthe fin, and a step of welding a next unit to the body with the heatconductive fin in the axial direction from an open side of the fin, andwelding contacting edges of adjacent ring sheets A together from theoutside.

[0048] According to this method of manufacturing the cask, first, thering sheet A and the heat conductive fins are welded to provide a unit,thereby to facilitate the welding work. Next, the units are welded tothe body with the heat conductive fins. As the cross section of the unithas a T shape, both sides of the fine are open, and therefore, it ispossible to carry out the welding work from the outside. Further, at thetime of welding the ring sheets A together, the welding is carried outfrom the outside. With this arrangement, it is not necessary to carryout the welding using an exclusive welding machine in a narrow and longspace like the conventional practice, and it is possible to carry outthe entire welding step from the outside. Therefore, it is possible toassemble the cask easily.

[0049] The method of manufacturing a cask of this invention comprises astep of forming a unit by welding a ring-shaped heat conductive finalong an approximately central part of an internal surface of a ringsheet A, a step of welding the unit to a body of the cask with the heatconductive fin in an axial direction of the cask from an open side ofthe fin, a step of accommodating approximately a half of a molded resinformed in a ring shape into the unit, and a step of welding a next unitto the body with a heat conductive fin in the axial direction from anopen side of the fin, welding contacting edges of adjacent ring sheets Atogether from the outside, and accommodating a remaining half of theformed molded resin in the unit.

[0050] According to this method of manufacturing the cask, the ringsheet A and the heat conductive fins are welded to provide a unit, andthe units are welded to the body with the heat conductive fins, in asimilar manner to the above. Therefore, it is possible to carry out thewelding work from the outside in an open status. Then, in a status thata molded resin formed in a ring shape is accommodated in the unit, anext unit is welded from the outside, the molded resin is accommodatedinto the unit, and the edges of adjacent ring sheet A are weldedtogether from the outside. With this arrangement, it is not necessary tocarry out welding using an exclusive welding machine in a narrow andlong space like the conventional practice, and it is possible to carryout the entire welding step from the outside. Therefore, it is possibleto assemble the cask easily.

[0051] The method of manufacturing a cask of this invention comprises astep of forming a unit by welding ring-shaped heat conductive fins alongboth sides of one ring sheet A with a margin left along edges of theboth sides, a step of welding at least two units to the body with theheat conductive fins in an axial direction of the cask from the outsideof the units by arranging the units at predetermined intervals, and astep of disposing another ring sheet B in a gap between the ring sheet Aand the ring sheet A of adjacent units and welding the ring sheet B fromthe outside.

[0052] According to this method of manufacturing the cask, first, thering sheet A and the heat conductive fins are welded to provide a unit,thereby to facilitate the welding work. Next, the units are welded tothe body with the heat conductive fins. As the cross section of the unithas a T shape, both sides of the fin are open, and therefore, it ispossible to carry out the welding work from the outside. Further, at thetime of welding the ring sheet B by covering it over a gap between thering sheets A and A, the welding is carried out from the outside. Withthis arrangement, it is not necessary to carry out the welding using anexclusive welding machine in a narrow and long space like theconventional practice, and it is possible to carry out the entirewelding step from the outside. Therefore, it is possible to assemble thecask easily.

[0053] The method of manufacturing a cask of this invention comprises astep of forming a unit by welding ring-shaped heat conductive fins alongboth sides of one ring sheet A with a margin left along edges of theboth sides, a step of accommodating a ring-shaped resin into the unit, astep of welding at least two units to a body of the cask with the heatconductive fins in an axial direction of the cask from the outside ofthe units by arranging the units at predetermined intervals, a step ofdisposing a resin molded in a ring-shape into between adjacent units,and a step of covering another ring sheet B over a gap between the ringsheet A and the ring sheet A of adjacent units and welding the ringsheet B from the outside.

[0054] According to this cask, the ring sheet A and the heat conductivefins are welded to provide a unit, and the units are welded to the bodywith the heat conductive fins, in a similar manner to the above.Therefore, it is possible to carry out the welding work from the outsidein an open status. At this time, the molded resin formed in a ring shapeis accommodated within the unit. While the molded resin is formed in aring shape, this may be in an integrated structure or in a split-typestructure obtained by dividing the resin in a circumferential directionof the cask. In the case of an integrated structure, the resin may bemolded within the unit, or the molded resin may be disposed in advanceat the time of welding the heat conductive fins. In the case of thesplit-type structure, the molded resin may be fixed to the body inadvance and accommodated in the unit at the time of welding the unit, ormay be sequentially accommodated into the units after the units areformed.

[0055] At least two units are welded to the body in an axial directionof the cask from the outside of the units at predetermined intervals. Atthis time, a resin is disposed in a similar manner to the above.Further, a molded resin is disposed between adjacent units. The ringsheet B is covered over a gap between the ring sheet A and the ringsheet A of the adjacent units and is welded from the outside. With thisarrangement, it is not necessary to carry out the welding using anexclusive welding machine in a narrow and long space like theconventional practice, and it is possible to carry out the entirewelding step from the outside. Further, the resin is easily arranged. Asa result, it is possible to assemble the cask easily.

[0056] In the method of manufacturing a cask of this invention based onthe method of manufacturing the cask, the unit and/or the ring sheet Bhas a structure obtained by being divided into parts in acircumferential direction of the cask, and these parts are individuallywelded to the body.

[0057] By dividing the unit and/or the ring sheet B in thecircumferential direction, it is possible to fix the unit and the ringsheet B to the body from its side face. Further, it is easy to arrange amolded resin within the unit and between the units. Therefore, it ispossible to assemble the cask easily.

[0058] The cask of this invention comprises a body internally providedwith a basket that accommodates radioactive materials such as spent fuelassemblies, and a plurality of sheet members provided around the body. Apart of each of the members is in contact with the body, another part ofthe member constitutes an external surface of the cask and forms asingle or plural angular-shaped portions or valley-shaped portions, andstill another part of the member is connected to an adjacent sheetmember, and a neutron absorber is filled in a space formed with thesheet members. Decay heat from the spent fuel assemblies is conductedfrom the body to the sheet members, and is radiated from the externalsurface of the cask. As the angular-shaped portion or the valley-shapedportion is formed on the external surface of the cask, the heatradiation area increases as compared with that of a conventional caskthat is simply formed in a cylindrical external shape. Consequently,heat radiation is efficiently performed. The sheet members may bemanufactured by bending plate materials, or may be manufactured byextrusion.

[0059] Particularly, by combining the neutron absorber with a honeycombmember, and by bringing the honeycomb member into contact at least withthe body, it is possible to carry out heat conduction via the honeycombmember even when the thermal conductivity of the neutron absorber islow. Further, by combining the neutron absorber with a plurality of heatconductors, it is also possible to improve the thermal conductivity.

[0060] The honeycomb member or the heat conductor is made of aluminum orcopper, and the sheet member is made of iron, copper, or aluminum. Withthis arrangement, it is possible to provide the honeycomb member or theheat conductor with a main heat conduction function, and provide thesheet members with strength. In order to further improve the thermalconductivity, it is possible to adhere a copper plate, an aluminumplate, or a graphite sheet onto a single side or both sides of the sheetmember. In this case, the use of the honeycomb member or the heatconductor is not essential. Further, it is also possible to improve thethermal conductivity by allowing a part of the sheet member contactingthe body or a part of the copper or aluminum plate to be in surfacecontact with the body.

[0061] This cask comprises a basket that is provided with cells wherespent fuel assemblies are accommodated and has an approximatelyoctagonal external shape, a body that accommodates the basket with itsexternal shape and internal shape formed to match the basket, andshields a gamma ray, and a neutron shield externally provided around thebody. With this arrangement, it is possible to ensure a thickness of thebody that is thick enough to shield the gamma ray, and it is possible tolighten the cask. Further, by provided the neutron shield so as to be anoctagonal shape with respect to the body, it is possible to reduce theneutron shield.

[0062] Further, the cask comprises a body internally provided with abasket that accommodates radioactive materials such as spent fuelassemblies, a plurality of sheet members provided around the body, witha part of each of the members being in contact with the body, anotherpart of the member constituting an external surface of the cask, andstill another part of the member being connected to an adjacent sheetmember, and a heat good conductor that is adhered to the sheet memberand has an extension section that reaches to contact the body.Therefore, it is possible to improve the thermal conductivity andradiate heat efficiently.

[0063] Furthermore, the cask comprises a body internally provided with abasket that accommodates radioactive materials such as spent fuelassemblies, and a plurality of sheet members provided around the body,with a part of each of the members being in contact with the body,another part of the member constituting an external surface of the cask,and still another part of the member being connected to an adjacentsheet member. The sheet member further includes a fitting section withthe body, and an intermediate member connected to the fitting section.The fitting section has higher weldability with respect to the body thanother portions of the sheet member, and the intermediate member hashigher thermal conductivity than other portions of the sheet member.Therefore, it is possible to improve weldability and improve heatconduction of the sheet members. Further, it is possible to improve thethermal conductivity, which makes it possible to radiate heatefficiently.

[0064] Furthermore, the cask comprises a body internally provided with abasket that accommodates radioactive materials such as spent fuelassemblies, and a plurality of sheet members provided around the body,with a part of each of the members being in contact with the body,another part of the member constituting an external surface of the cask,and still another part of the member being connected to an adjacentsheet member. The sheet member includes a portion that functions as aheat conductive fin and a portion that functions as an externalcylinder, the portions being made of different materials from eachother. Thermal conductivity of the portion that functions as the heatconductive fin is relatively high, and heat resistance of the portionthat functions as the external cylinder is relatively high. Therefore,it is possible to improve the thermal conductivity and radiate heatefficiently. Further, it is possible to provide heat resistance to themembers.

[0065] Furthermore, the cask comprises a body internally provided with abasket that accommodates radioactive materials such as spent fuelassemblies, and a plurality of sheet members provided around the body,with a part of each of the members being in contact with the body,another part of the member constituting an external surface of the cask,and still another part of the member being connected to an adjacentsheet member. The sheet member further includes a fitting section withthe body, and the fitting section has higher weldability with respect tothe body than other portions of the sheet member. Therefore, it ispossible to improve the thermal conductivity, and it becomes possible toeasily weld the sheet members to the body.

BRIEF DESCRIPTION OF THE DRAWINGS

[0066]FIG. 1 is a perspective view which shows a cask according to afirst embodiment of this invention,

[0067]FIG. 2 is a cross-sectional view of the cask shown in FIG. 1 inits radial direction,

[0068]FIG. 3 is an explanatory view which shows a structure of a basket,

[0069]FIG. 4 is an explanatory view which shows a structure of a basket,

[0070]FIG. 5 is an explanatory view which shows a structure of a basket,

[0071]FIG. 6 is a detailed explanatory view which shows a structure ofan external cylinder and heat conductive fins,

[0072]FIG. 7 is an enlarged view of a welded portion shown in FIG. 6,

[0073]FIG. 8 is a flowchart which shows a method of manufacturing sheetmembers that constitute a basket, and

[0074]FIG. 9 is a schematic perspective view which shows a machiningapparatus of a cavity.

[0075]FIG. 10 is a schematic explanatory view which shows a method ofmachining a cavity,

[0076]FIG. 11 is an explanatory view which shows an assembly procedureof an external cylinder and heat conductive fins,

[0077]FIG. 12 is an explanatory view which shows a modification of theexternal cylinder and heat conductive fins,

[0078]FIG. 13 is an explanatory view which shows a modification of theexternal cylinder and heat conductive fins,

[0079]FIG. 14 is an explanatory view which shows a modification of theexternal cylinder and heat conductive fins,

[0080]FIG. 15 is an explanatory view of a cask according to a secondembodiment of this invention,

[0081]FIG. 16 is a modification of the cask shown in FIG. 15,

[0082]FIG. 17 is an explanatory view which shows a cask according to athird embodiment of this invention,

[0083]FIG. 18 is an explanatory view which shows a modification of thecask shown in FIG. 17, and

[0084]FIG. 19 is an explanatory view which shows a modification of thecask shown in FIG. 17.

[0085]FIG. 20 is a top plan view which shows an external cylinder of acask according to a fourth embodiment of this invention,

[0086]FIG. 21 is a top plan view which shows a modification of theexternal cylinder shown in FIG. 20,

[0087]FIG. 22 is an assembly view which shows a cask according to afifth embodiment of this invention,

[0088]FIG. 23 is an A-A cross-sectional view shown in FIG. 22,

[0089]FIG. 24 is an assembly view which shows a modification accordingto the fifth embodiment of this invention,

[0090]FIG. 25 is a cross-sectional view of a part of the cask shown inFIG. 24,

[0091]FIG. 26 is a perspective view which shows a cask according to asixth embodiment of this invention,

[0092]FIG. 27 is a cross-sectional view of the cask shown in FIG. 26 inits radial direction,

[0093]FIG. 28 is a partially enlarged view shown in FIG. 27, and

[0094]FIG. 29 is an explanatory view which shows a honeycomb member anda heat conductive material.

[0095]FIG. 30 is an explanatory view which shows sheet members,

[0096]FIG. 31 is an explanatory view which shows a modification of thiscask,

[0097]FIG. 32 is an explanatory view which shows the modification of thecask shown in FIG. 31,

[0098]FIG. 33 is an explanatory view which shows the modification of thecask shown in FIG. 31,

[0099]FIG. 34 is an explanatory view which shows the modification of thecask shown in FIG. 31,

[0100]FIG. 35 is a partially enlarged view of the view shown in FIG. 34,

[0101] FIG. 36 is an explanatory view which shows the modification ofthe cask shown in FIG. 31,

[0102]FIG. 37 is an explanatory view which shows a modification of thesheet members,

[0103]FIG. 38 is an explanatory view which shows a modification of sheetmembers, and

[0104]FIG. 39 is a perspective view which shows one example of aconventional cask.

[0105]FIG. 40 is a cross-sectional view of the cask shown in FIG. 39 inits axial direction,

[0106]FIG. 41 is the structural diagram which shows one example of theconventional cask, and

[0107]FIG. 42 is a cross-sectional view of the cask shown in FIG. 41 inits axial direction.

BEST MODE FOR CARRYING OUT THE INVENTION

[0108] Embodiments of the cask according to this invention will beexplained in detail below with reference to the drawings. This inventionis not limited by these embodiments. Further, it is needless to mentionthat constituent elements of this invention include those that personsskilled in the art can easily assume.

[0109] First Embodiment:

[0110]FIG. 1 is a perspective view which shows a cask according to thefirst embodiment of this invention. FIG. 2 is a cross-sectional view ofthe cask shown in FIG. 1 in its radial direction. A cask 100 accordingto this first embodiment is machined so as to have the internal surfaceof a cavity 102 of a body 101 matched with an external peripheral shapeof a basket 130. The internal surface of the cavity 102 is machined bymilling with an exclusive machining apparatus to be described later. Thebody 101 and a bottom plate 104 are forged using carbon steel having agamma-ray shielding function. It is possible to use stainless steelinstead of carbon steel. The body 101 and the bottom plate 104 areconnected together by welding. In order to secure sealing performance ofa pressure-resistant vessel, a metallic gasket is provided between aprimary lid 110 and the body 101.

[0111] A resin 106 that is a polymer material containing much hydrogenand has a neutron shielding function is filled between the body 101 andan external cylinder 105. Further, a plurality of copper heat conductivefins 107 that carry out heat conduction are welded between the body 101and the external cylinder 105. The resin 106 is injected in a fluidstatus into each space formed between the heat conductive fins 107, andis cooled and hardened. A honeycomb unit made of aluminum or copper maybe disposed within the space, and a neutron shield may be charged intothe honeycomb by pressure (not shown). It is preferable that the heatconductive fins 107 are provided in high density in sections having ahigh quantity of heat in order to radiate heat uniformly. An allowance108 for heat expansion of a few millimeters is provided between theresin 106 and the external cylinder 105.

[0112] A lid section 109 is constructed of a primary lid 110 and asecondary lid 111. This primary lid 110 is a disk made of stainlesssteel or carbon steel that shields the gamma ray. The secondary lid 111is also a disk made of stainless steel or carbon steel that shields thegamma ray. A resin 112 is sealed as a neutron shield in the uppersurface of this secondary lid 111. The primary lid 110 and the secondarylid 111 are fitted to the body 101 with bolts made of stainless orcarbon steel. Further, a metallic gasket is proved between the primarylid 110 and the secondary lid 111 and the body 101 respectively, therebyto hold internal air tightness of the cask. At both sides of the caskbody, trunnions 117 are provided to suspend the cask 100.

[0113] The internal shape of the cavity 102 matches the external shapeof the basket 130. FIG. 3 is an explanatory view which shows a structureof the basket. This basket 301 is structured by providing a cut section312 in a belt-like sheet member 310 having a through hole 311, and byintersecting the sheet members 310 at right angles to alternatelysuperposing the intersecting members on each other. Based on thisstructure, cells 307 that accommodate spent fuel assemblies are formed.The through hole 311 is formed in a character B-like shape in its crosssection in a longitudinal direction of the sheet member 310, and aplurality of communicating holes (not shown) are formed in a central rib313 between the through holes. The through hole 311 is communicated tothe through hole 311 of another sheet member 310 at the cut section 312.Further, on the end surface of the sheet member 310 in its longitudinaldirection, a communicating hole 314 is provided to communicate betweenthe through holes 311 of upper and lower sheet members 310.

[0114] A concave section 315 and a convex section 316 are formed atupper and lower edges of the sheet member 310. The upper and lower sheetmembers 310 are positioned together with the concave section 315 and theconvex section 316 (refer to FIG. 4). This avoids the occurrence ofstages within the cells 307. Therefore, it is possible to smoothlyaccommodate the spent fuel assemblies within the cells 307. Further, aconvex section 317 is formed at the edge of the sheet member 310.

[0115] As a stage is formed at the edge of the sheet member 310 byproviding the convex section 317, a heat conductive plate 318 is coveredover a gap between the adjacent stages, as shown in FIG. 5(a). Based onthis, the external peripheral surface of the basket 130 is formed.Aluminum or aluminum alloy added with boron is used for the material ofthe sheet members 310 and the heat conductive plates 318. A system offitting the heat conductive plates 318 is not limited to the system ofproviding the convex sections 317 as shown in this drawing. For example,the heat conductive plates 318 may be fixed by spot welding by applyingthe heat conductive plates 318 over the edges of the sheet members 310(not shown). Further, the heat conductive plates 318 may be fitted foreach sheet member 310 as shown in FIG. 5(b), instead of providing theheat conductive plates 318 over the upper and lower sheet members 310.With this arrangement, it is possible to form the basket 130 byproviding the sheet member 310 as a small assembling block for eachstage and laminating these blocks together, as shown in FIG. 5(c).Therefore, the blocks can be easily inserted into the cavity while thebasket 130 can be also easily extracted.

[0116] The external shape of the basket 130 becomes such that its foursurfaces 301 a are formed in one plane according to the heat conductiveplates 318, and other four surfaces 301 b are formed in angularcross-sectional shape. The internal shape of the cavity 102 is formed inone plane so as to keep in approximately close contact with the oneplane section (301 a) of the basket 130. A section corresponding to theangular cross-sectional section (301 b) of the basket approximatelymatches this shape, by leaving a space S at each corner section. Next, adummy pipe 318 having a triangle in its cross section is inserted intothis space S. With this dummy pipe 308, it becomes possible to reducethe weight of the body 302 and make uniform the thickness of the body302. Further, it is possible to insure a good fit of the basket 130 withrespect to the cavity by minimizing rattle.

[0117] When the inside of the dummy pipe 308 is sealed, pure water isprevented from permeating the dummy pipe 308 when the pure water ispoured into a storage facility. Therefore, there is an effect ofreducing the weight of the cask. Further, by sealing the inside of thedummy pipe 308, it is also possible to fill other material into theinside. For example, by filling helium gas into the inside in advance,it is possible to facilitate the helium gas introduction work when thecask is stored. Further, by sealing helium gas inside, it is possible toimprove the thermal conductivity during the storage time. When heliumgas is to be introduced, it is preferable to provide a valve on one ofthe lids.

[0118] The trunnions 117 are fixed directly to the body 101. In thiscase, the trunnions 117 are preferably fixed to the angularcross-sectional sections 301 b of the body 101. As the angularcross-sectional sections 301 b have slightly larger allowance than theone plane section 301 a in the thickness of the body 101, there is smallinfluence from the viewpoint of shielding the gamma ray even when thetrunnion base is machined. While the resin 117 a is filled in thetrunnions 117, it is possible to prevent the transmission of neutronfrom sections 117 b of the trunnions 117 where no resin is filled, byfilling the resin into the dummy pipes 308 provided in the space S.

[0119]FIG. 6 is a detailed explanatory view which shows a structure ofthe external cylinder and heat conductive fins. This external cylinder105 has a split-type structure consisting of belt-like members 105 a and105 b. Both edges of the belt-like member 105 a are left as they are,and the heat conductive fins 107 are welded at both sides of thebelt-like member 105 a (welded joints 205) except for the both edges,and have their cross sections in a gateway shape. The heat conductivefins 107 are welded to bulge portions 201 on the surface of the body 101(welded joints 206). The bulge portions 201 are made of iron, and thisimproves weldability with the heat conductive fins 107 made of copper.The belt-like member 105 a and the heat conductive fins 107 and 107 areformed in a unit 105 c having a U-shaped cross section, and these unitsare welded on the surface of the body 101 at predetermined intervals.

[0120]FIG. 7 is an enlarged view of the welded portions shown in FIG. 6.Stage sections 202 are formed at both edges of the belt-like member 105a. A stage section 203 of the belt-like member 105 b is engaged with oneof the stage sections 202, and is welded (welded joints 204). Further,as is clear from this drawing, the welded joints 204 between thebelt-like members 105 a and 105 b and the welded joints 205 between theheat conductive fin 107 and the belt-like member 105 a are spaced fromeach other to some extent. With this arrangement, it is possible toprevent local concentration of thermally influenced sections.Accordingly, it is possible to prevent a thermal deformation of theexternal cylinder 105 and the heat conductive fins 107, and to relaxinternal stress. For the welding, a conventional welding method such asthe TIG welding and the MIG welding can be used. A honeycomb member 207made of aluminum is fixed to the inside of the external cylinder 105 toform a void layer.

[0121]FIG. 8 is a flowchart which shows a method of manufacturing sheetmembers that constitute the basket. First, powder of Al or Al alloy ismanufactured using a rapid freezing method like the atomizing method(step S401), powder of B or B compound is prepared (step S402), andthese particles are mixed for 10 to 15 minutes with a cross rotary mixeror the like (step S403).

[0122] For the Al or Al alloy, it is possible to use pure aluminumground metal, Al—Cu base aluminum alloy, Al—Mg base aluminum alloy,Al—Mg—Si base aluminum alloy, Al-Zn-Mg base aluminum alloy, and Al-Febase aluminum alloy. For the above B and B compound, it is possible touse B4C, B2O3, or the like. It is preferable that the quantity of boronadded to aluminum is not less than 1.5 weight % and not more than 7weight %. This is because it is not possible to obtain sufficientneutron absorbing performance at 1.5 weight % or below, and stretchbased on tension is lowered at 7 weight % or above.

[0123] Mixed powder is sealed into a rubber case, and a high pressure isapplied uniformly from all directions at a normal temperature accordingto the CIP (Cold Isostatic Press), thereby to carry out a powder molding(step S404). Molding conditions of the CIP are set such that the moldingpressure is 200 MPa, the diameter of the molded product is 600 mm, andthe length is 1500 mm. By uniformly applying pressure from alldirections according to the CIP, it is possible to obtain a high-densitymolded product with small variations in the molding density.

[0124] The powder-molded product is sealed into a can in vacuum, and thetemperature is raised to 300° C. (step S405). A gas component and awater component within the can are removed in this degassing step. Atthe next step, the vacuum degassed molded product is remolded accordingto the HIP (Hot Isostatic Press) (step S406). Molding conditions of theHIP are set such that the temperatures are 400° C. to 450° C., the timeis 30 sec, the pressure is 6000 tons, and the diameter of the moldedproduct is 400 mm. Next, in order to remove the can, the outside cuttingand the end-surface cutting are carried out (step S407), and a billet asthe molded product is hot excluded using a porthole extruder (step S408)Extrusion conditions in this case are set such that the heatingtemperatures are 500° C. to 520° C., and the extrusion speed is 5 m/min.These conditions are appropriately changed depending on the containedquantity of B. Next, after the extrusion, the tension is corrected (stepS409), and non-stationary sections and evaluation sections are cut, toobtain the sheet member 310 (step S410). Next, a plurality of cutsections 312 are formed on the sheet member 310 by machining (stepS411).

[0125] The machining of the cavity 102 of the body 101 will be explainedbelow. FIG. 9 is a schematic perspective view which shows a machiningapparatus of the cavity 102. This machining apparatus 140 comprises afixed table 141 that extends along the inside of the body 101 and ismounted and fixed within the cavity 102, a movable table 142 that slideson the fixed table 141 in an axial direction, a saddle 143 that ispositioned and fixed on the movable table 142, a spindle unit 146 thatis provided on the saddle 143 and includes a spindle 144 and a drivingmotor 145, and a face mill 147 provided on a spindle axis. A reactionforce receiver 148 that has a contact section formed along the internalshape of the cavity 102 is provided on the spindle unit 146. Thisreaction force receiver 148 is detachable and slides along a dovetailgroove (not shown) in arrow directions in the figure. The reaction forcereceiver 148 has a clamp unit 149 for the spindle unit 146, and can befixed at a predetermined position.

[0126] A plurality of clamp units 150 are fixed to within a lower grooveof the fixed table 141. Each of the clamp unit 150 is composed of ahydraulic cylinder 151, a wedge-like moving block 152 provided on theshaft of the hydraulic cylinder 151, and a fixed block 153 that isbrought into contact with the moving block 152 on a sloped surface. Theclamp unit 150 is fixed at its shaded side in the figure, to theinternal side of the groove of the fixed table 141. When the shaft ofthe hydraulic cylinder 151 is driven, the moving block 152 is broughtinto contact with the fixed block 153, and moves slightly downward dueto the effect of wedge (shown by a dotted line in the figure). With thisarrangement, the lower surface of the moving block 152 is pressedagainst the internal surface of the cavity 102. Therefore, the fixedtable 141 can be fixed within the cavity 102.

[0127] The body 101 is mounted on a rotary supporting base 154 made of aroller, and is rotatable in a radial direction of the body 101. Further,by fitting a spacer 155 into between the spindle unit 146 and the saddle143, it is possible to adjust the height of the face mill 147 on thefixed table 141. The thickness of the spacer 155 is approximately thesame as-the size of each cell. The saddle 143 moves in a radialdirection of the body 101 when a handle 156 provided on the movabletable 142 is turned. Movement of the movable table 142 is controlled bya servomotor 157 provided at the end of the fixed table 141 and a ballscrew 158. As the machining proceeds, the shape inside the cavity 102changes. Therefore, the shapes of the reaction force receiver 148 andthe moving block 152 of the clamp unit 150 need to be changed tosuitable ones.

[0128]FIG. 10 is a schematic explanatory view which shows the method ofmachining a cavity. First, the fixed table 141 is fixed at apredetermined position within the cavity 102 by the clamp units 150 andthe reaction force receiver 148. Next, as shown in FIG. 10(a), thespindle unit 146 is moved along the fixed table 141 at a predeterminedcutting speed, and the inside of the cavity 102 is cut by the face mill147. After the cutting is completed at this position, the clamp units150 are removed, and the fixed table 141 is released. Next, as shown inFIG. 10(b), the body 101 is turned by 90 degrees on the rotarysupporting base 154, and the fixed table 141 is fixed by the clamp units150. Then, the cutting is carried out by the face mill 147 in the samemanner as explained above. Thereafter, the machining steps are furtherrepeated two times.

[0129] The spindle unit 146 is turned by 180 degrees, and the inside ofthe cavity 102 is cut sequentially, as shown in FIG. 10(c). In thiscase, the machining is repeated while turning the body 101 by 90 degreesin the same manner as the above. Next, the spacer 155 is engaged withthe spindle unit 146 to increase the height of the spindle unit, asshown in FIG. 10(d). Then, the face mill 147 is sent to an axialdirection keeping the same height, and cutting is carried out along theinside of the cavity 102. This step is repeated while turning the cavityby 90 degrees, thereby to substantially complete a shape required forinsertion of the basket 130. The cutting of the section into which thedummy pipe 308 is inserted may be carried out in the same manner as thatshown in FIG. 10(d). However, a spacer thickness to adjust the height ofthe spindle unit 146 is set to be equal to one side of the dummy pipe308.

[0130]FIG. 11 is an explanatory view which shows an assembly procedureof the external cylinder and heat conductive fins. First, as shown inFIG. 11(a), the belt-like member 105 a is placed on the work plane, andthe heat conductive fins 107 are erected on both sides of the belt-likemember to support this belt-like member. At this time, the belt-likemember 105 a have a margin left along its both side edges outside fromthe fins 107. Based on a width of the space, the interval between thewelded joints 204 and 205 (refer to FIG. 7) is determined. Next, asshown in FIG. 11(b), a worker W carries out the welding from the outsideof the heat conductive fins 107, and form these elements as a unit 105 chaving a U shape in its cross section. The unit 105 c is prepared inadvance by a number that is required for disposing the unit 105 c aroundthe body 101 at a predetermined interval.

[0131] As shown in FIGS. 11(c) and 11(d), the unit 105 c is welded tothe body 101. The unit 105 c is welded by fitting the heat conductivefins 107 to the bulge portions 201. At this time, the worker W manuallycarries out the welding from only the outside of the unit 105 c, anddoes not carry out the welding from the inside of the unit 105 c. Evenif adjacent units 105 c have been welded in advance, these units arewelded at predetermined intervals, and therefore it is possible to carryout the welding by inserting a torch or an electrode bar (T) from thegap L. The body 101 is supported by a rotary supporting base 208 and atool 209, and it is therefore possible to weld the unit 105 c whileturning the body 101.

[0132] As shown in FIGS. 11(e) and 11(f), another belt-like member 105 bis covered over a gap between the adjacent belt-like members 105 a and105 a, and the stage sections 203 of the member 105 b are engaged withthe adjacent stage sections 202 of the members 105 a. The worker W weldsthe surface-side boundary between the belt-like member 105 a and thebelt-like member 105 b (the welded joints 204). At this time, thewelding is not carried out from the backside of the boundary. By weldingall boundaries between the belt-like member 105 b and the adjacentbelt-like members 105 a, the external cylinder 105 of the cask 100 isfinished.

[0133] When the external cylinder 105 and the heat conductive fins 107are assembled, it is not necessary to carry out the welding within anarrow and long space formed by the external cylinder 105 and the heatconductive fins 107. In other words, in the above reference example, theself-run type welding machine is run within the space formed between theexternal cylinder 503 and the heat conductive fins 508 to carry out thewelding. However, when the external cylinder 105 and the heat conductivefins 107 of the above structures are assembled according to the aboveprocedure, it becomes possible to carry out the whole of the weldingstep from the outside, and it is possible to assemble the cask using anordinary welding facility without an exclusive welding machine.Therefore, the cask assembling work becomes extremely easy.

[0134] Referring to FIG. 1 and FIG. 2, the spent fuel assemblies thatare accommodated in the cask 100 contain fissionable materials andfission products, and generate radiation with decay heat. Therefore, itis necessary to securely maintain the heat removing function, theshielding function, and the critical-state preventing function duringthe storage period (about 60 years). According to the cask 100 of thefirst embodiment, the inside of the cavity 102 of the body 101 ismachined to insert the basket 130 so as to keep the external peripheralsurface of the basket 130 in close contact with the cavity 102(substantially no space) Further, the heat conductive fins 107 areprovided between the body 101 and the external cylinder 105. Therefore,the heat from the fuel bar is conducted to the body 101 through thebasket or the filled helium gas, and is discharged from the externalcylinder 105 mainly through the heat conductive fins 107. From theabove, it is possible to improve the thermal conductivity from thebasket 130, and it becomes possible to efficiently remove the decayheat.

[0135] The gamma ray generated from the spent fuel assemblies isshielded by the body 101, the external cylinder 105, and the lid section109, which are made of carbon steel or stainless steel. Further, neutronis shielded by the resin 106 to avoid the influence of exposure toradiation operators. Specifically, it is designed to obtain a shieldingfunction such that a surface dose equivalent rate is 2 mSv/h or below,and a dose equivalent rate at 1 m deep from the surface is 100 μSv/h orbelow. Further, as aluminum alloy containing boron is used for the sheetmembers 310 that constitute the cells to absorb neutron, it is possibleto prevent the spent fuel assemblies from reaching a critical state.

[0136] As explained above, according to the cask 100 of the firstembodiment, the external cylinder 105 has the split-type structure, anda plurality of units 105 c are manufactured each consisting of thebelt-like member 105 a and the heat conductive fins 107. These units arewelded to the body 101 from the outside at predetermined intervals. Atthe same time, the belt-like members 105 b are covered over the wholegaps each between the adjacent belt-like members 105 a, and are weldedfrom the outside. Therefore, the welding in a narrow and long space isnot necessary. Consequently, it is possible to carry out the weldingwork easily, and any special exclusive welding machine is not necessary.Further, as it is possible to assemble the cask 100 in widely usedwelding facilities, it is possible to carryout the assembling easily inthe majority of companies.

[0137] Since the inside of the cavity 102 of the body 101 is machined toinsert the basket 130 so as to keep the external peripheral surface ofthe basket 130 in close contact with the cavity, it is possible toimprove the thermal conductivity from the basket 130. Particularly, thedecay heat is efficiently transmitted to the body 101 via the heatconductive plates 318 that are provided on the external peripheralsurface of the basket. Further, as a part of the angular cross section301 b of the basket 130 is in surface contact with the body 101, thiscontributes to secure holding of the basket 130 and improvement of theheat conduction efficiency. Further, by inserting the dummy pipe 308 inthe space S, the heat conduction efficiency is more improved. It isneedless to mention that in the above structure, it is possible toimprove the thermal conductivity to some extent even if the heatconductive plates 318 are omitted.

[0138] Since it is possible to eliminate the space within the cavity102, the body 101 can be made compact and lightweight. Even in thiscase, the number of spent fuel assemblies to be accommodated is notdecreased. On the contrary, when the external diameter of the body 101is made equal to that of the cask 500 shown in FIG. 26, it is possibleto ensure even larger number of cells by the amount. Therefore, it ispossible to increase the number of spent fuel assemblies to beaccommodated. Specifically, the cask 100 can accommodate 37 spent fuelassemblies, but the external diameter of the body of the cask can berestricted to 2560 mm and the weight thereof to 120 tons.

[0139] Modification of the external cylinder and the heat conductivefins:

[0140]FIG. 12 is an explanatory view which shows a modification of theexternal cylinder and the heat conductive fins. As shown, heatconductive fins 210 may be inclined. The structures of the otherelements are the same as those of the above cask. By diagonallydisposing the heat conductive fins 210, the existence of the resin 106in a direction in which neutrons are discharged from the spent fuelassemblies (arrow marks in the figure) prevents the neutrons fromleaking to the outside through the heat conductive fins 210. While theheat conductive fins 210 are inclined in alternate directions as shownin the figure, it is also possible to dispose all heat conductive finsin a radial direction from the center of the container. Alternatively,the heat conductive fins 210 may be slanted in one direction (notshown).

[0141] As shown in FIG. 13, a belt-like member 211 b may be welded insuper imposition at edges of belt-like members 211 a. In this case, thebelt-like member 211 a and heat conductive fins 107 are formed in a unithaving a U-shaped cross section, and these units are welded to a body101 respectively, and the belt-like members 211 b are covered over thewhole gaps each between the adjacent belt-like members 211 a, and arewelded. In this structure, it is also possible to assemble the caskeasily in a similar manner and to prevent local concentration ofthermally influenced sections, by separating welded joints 212 fromwelded joints 213. Further, as it is possible to omit stage preparationand beveling at the edges of the belt-like members, it is possible tosimplify the structure of the external cylinder 211.

[0142] In the structure shown in FIG. 14, a heat conductive fin 214 isbent in a U shape in its cross sections, and the belt-like member 105 ais mounted on the upper surface of the heat conductive fin 214. Angularedge sections of the heat conductive fin 214 are welded with thebelt-like member 105 a (welded joints 215). With this arrangement, sincethe welding is carried out by mounting the heat conductive fins 214 onthe belt-like members 105 a, it becomes easy to carry out theassembling. Further, like in the above cases, it is possible to carryout the entire welding step from the outside, without welding in anarrow and long space. Therefore, it is possible to assemble the caskeasily. Further, by separating the welded joints 204 from the weldedjoints 215, it is possible to prevent local concentration of thermallyinfluenced sections. Further, as the heat conductive fins 214 made ofcopper are in surface contact with the belt-like members 105 a, it ispossible to efficiently transmit the decay heat from the inside of thecask to the external cylinder 105 and radiate the heat to the outside.In FIG. 14, a dashed line shows a heat radiation route.

[0143] Second Embodiment:

[0144]FIG. 15 is an explanatory view of a cask according to the secondembodiment of this invention. This cask is characterized in that anexternal cylinder 216 has an angular shape in its cross section. Heatconductive fins 107 are welded to both edges of a belt-like member 216 athat is formed in an angular shape by bending (welded joints 205),thereby to form a unit. These units are welded onto bulge portions 201of the body 101 (welded joints 206). Further, belt-like member 216 b isformed by bending in the similar manner. The stage section 203 of thebelt-like member 216 b is engaged with the stage section 202 of thebelt-like member 216 a and is welded from the outside (welded joints204). Honeycomb members are provided on the internal surfaces of thebelt-like members 216 a and b. A resin that absorbs neutron is filled inspaces formed with the belt-like members 216 a or 216 b and the heatconductive fins 107.

[0145] In the above structure, it is possible to carry out the entirewelding step from the outside, and therefore it is also possible toassemble the cask in the similar manner to the above. Further, byseparating the welded joints 204 from the welded joints 205, it ispossible to prevent local concentration of thermally influencedsections. Further, as the belt-like members 216 a and b are in anangular shape, the surface area of the external cylinder 216 becomeslarge, which makes it possible to improve the heat radiation effect.Further, when the cask has a plurality of sheet-like heat radiation finsstretched to the external cylinder, it is difficult to clean the cornersbetween the heat radiation fins and the external cylinder. However, inthe case of the angular external cylinder 216, there is no narrowcorner. Therefore, this has an advantage that it is easy to clean thesurface of the external cylinder. When the external cylinder has avalley shape instead of the angular shape, there is the same effect asexplained above (not shown).

[0146] As shown in FIG. 16, the edge of belt-like member 217 and theedge of the adjacent belt-like member 217 are butted on the top of theedge of the heat conductive fin 107, and the butt sections are welded(welded joints 219). The ends of the belt-like members 217 are beveledin advance. By preparing the external cylinder 218 in an angular shape,it is possible to improve the heat radiation performance of the cask.

[0147] Third Embodiment:

[0148]FIG. 17 is an explanatory view which shows a cask according to thethird embodiment of this invention. An external cylinder 220 of thiscask is characterized in that a unit 220 a having a T shape in its crosssection is prepared by welding (welded joints 222) the heat conductivefin 107 at the center of a belt-like member 105 a. The welding procedureis approximately similar to that of the first embodiment. First, theheat conductive fin 107 is welded to the belt-like member 105 a toprovide the unit 220 a. The welding of the heat conductive fin 107 andthe belt-like member 105 a may be carried out from one side, or may becarried out from both sides in order to secure strength (welded joints222). Next, these units 220 a are welded to the body 101 atpredetermined intervals. The body 101 is provided with a bulge portion201, and the heat conductive fin 107 is welded on the bulge portion 201.Further, the welding is carried out from both sides of the heatconductive fin 107 in order to secure strength. Then, belt-like members105 b are fitted into all the gaps each between the adjacent belt-likemembers 105 a, and are welded (welded joints 204).

[0149] In this structure, it is possible to carry out the entire weldingwork from the outside, without welding in a narrow and long space.Therefore, it is possible to assemble the cask easily in the same manneras the above. Further, by forming the cross section in the T shape, itis possible to provide a more distance between the welded joints 222 and204 than the distance provided in the first and second embodiments.Therefore, it is possible to prevent local concentration of thermallyinfluenced sections. Further, as shown in FIG. 18, the edges of heatconductive fins 224 may be bent in an L shape in their cross sections,and may be in surface contact with the belt-like members 105 a. In thiscase, in welding of the heat conductive fins 224 to the belt-likemembers 105 a, the belt-like member 105 a is welded to a corner edge ofthe heat conductive fin 224 (welded joints 223).

[0150] With the above arrangement, since the welding can be carried outby mounting the heat conductive fins 224 on the belt-like members 105 a,it becomes easy to carry out the assembling. Further, as the heatconductive fins 224 made of copper are in surface contact with thebelt-like members 105 a, it is possible to efficiently transmit thedecay heat from the inside of the cask to the external cylinder 220 andradiate the heat to the outside. In the figure, a dashed line shows aheat radiation route.

[0151]FIG. 19 is an explanatory view which shows a modification of thecask shown in FIG. 17. According to this cask, an external cylinder 225is constructed of belt-like members 226, each being welded to a heatconductive fin 107 at the center of the member 226 (welded joints 227)and formed in a unit 228 having a T shape in its cross section. Forassembling this external cylinder 225, the heat conductive fin 107 of aunit 228 a is first welded to the body 101 from one side (welded joints229 a). Next, the heat conductive fin 107 of a unit 228 b is welded tothe body 101 from one side that is open (welded joints 229 b), and thebelt-like members 226 a and 226 b are welded together (welded joints 230a).

[0152] Likewise, the heat conductive fin 107 of a unit 228 c is weldedto the body 101 from one side that is open (welded joints 229 c), andthe belt-like members 226 b and 226 c are welded together (welded joints230 b). The heat conductive fin 107 of a unit 228 d is also welded tothe body 101 at welded joints 229 d, and the belt-like members 226 c and226 d are welded together at welded joints 230 c. With the abovearrangement, it is also possible to carry out the entire welding workfrom the outside, and the welding in a narrow and long space is notnecessary. Therefore, the cask assembling work becomes extremely easy.Further, as the welded joints 230 are sufficiently separated from thewelded joints 227, it is possible to prevent local concentration ofthermally influenced sections.

[0153] Fourth Embodiment:

[0154]FIG. 20 is a top plan view which shows an external cylinder of acask-according to the fourth embodiment of this invention. While theexternal cylinder is formed by welding the belt-like members forconnection in the above embodiment, belt-like members 105 a thatconstitute an external cylinder 105 may be connected at ends of the caskin its axial direction. Four heat conductive fins 107 (shown by a dottedline in the figure) are welded to one sheet member 231 at predeterminedintervals. Further, the belt-like members 105 a are welded to the bodytogether with the heat conductive fins 107 at predetermined intervals. Abelt-like member 105 d is fitted in a hollow section 105 d and in a gapbetween the adjacent sheet members 231, and is welded respectively. Asthe heat conductive fin 107 is not disposed at a position opposite to acoupling section 233, this does not interfere with the welding work.

[0155] As shown in FIG. 21, by forming a sheet member 234 in a U shape,the adjacent sheet members 234 may be coupled to each other via acoupling section 235. In this structure, the belt-like member 105 b isfitted in a hollow section 236 formed on the sheet member 234, and iswelded. With this arrangement, an external cylinder 237 is formed.

[0156] Fifth Embodiment:

[0157]FIG. 22 is an assembly view which shows a cask according to thefifth embodiment of this invention. FIG. 23 is a cross-sectional view ofthe cask taken along the line A-A shown in FIG. 22. This cask 400 isprovided by forming units 402 of an external cylinder 401 in a ringshape, and by sequentially inserting these units into the body 101 inits radial direction and by welding the units. The ring-shaped unit 402has a structure that a belt-like plate is formed into a ring-shapedplate 403, and a ring-shaped heat conductive fin 404 is welded to theinternal peripheral surface of the plate 403. (welded joints 405). Thisunit 402 is fixed by welding its heat conductive fin 404 to a bulgeportion 406 provided on a circumferential direction of the body 101.These units 402 are fitted to the body 101, and peripheral edges of thering sheets 403 of adjacent units 402 are welded together (welded joints407). A ring-shaped resin 408 cured in advance with a mold is disposedin a space formed with these peripheral edges of the ring sheets 403that are welded together.

[0158] In assembly of the cask 400, a first unit 402 a is inserted intothe body 101, and the heat conductive fin 404 of the unit is welded tothe bulge portion 406 of the body 101. Next, a ring-shaped resin 408 ais inserted into the body 101, and about a half of the resin isaccommodated into the unit 402 a that has been already welded and fixed.Then, the next unit 402 b is inserted into the body 101, and about ahalf of the resin 408 a is accommodated into the inside of the body. Thering sheet 403 of the first unit 402 a and the ring sheet 403 of thenext unit 402 b are welded together (welded joints 407). At this time,the welding of the heat conductive fin 404 of the unit 402 b is carriedout from only an open side (welded joints 409). This operation isrepeated by a predetermined number of times. The assembling of theexternal cylinder 401 is finished when the last unit 402 has beenfitted. When it is difficult to insert the ring-shaped resin 408, theresin 408 may be divided into some pieces in a circumferentialdirection, and arc-shaped resins 408 x may be accommodated into thespace. Lids 410 are provided at both sides of the external cylinder 401.

[0159] Based on the above structure, it is possible to carry out all thewelding work from the outside, without welding in a narrow and longspace. Therefore, it is possible to carry out the welding easily, and aspecific exclusive welding machine is not necessary. Further, as it ispossible to assemble the cask 400 in widely used welding facilities, itis possible to carry out the assembling easily in the majority ofcompanies. Further, as the welded joints 405 are separated from thewelded joints 407, it is possible to prevent local concentration ofthermally influenced sections.

[0160]FIG. 24 is an assembly view which shows a modification accordingto the fifth embodiment of this invention. FIG. 25 is a cross-sectionalview of a part of the cask shown in FIG. 24. This cask 450 ischaracterized in that a unit 452 of an external cylinder 451 is dividedinto two arc-shaped sections. Heat conductive fins 454 are welded toedges of the internal surfaces of an arc plate 453, by leaving a slightmargin respectively (welded joints 455), thereby to form one unit 452.These units 452 are welded to the body 101 at predetermined intervals.Further, an arc plate 456 is fitted in a gap between the arc plate 453and the arc plate 453 of the unit 452, and is welded (welded joints457). Further, the units 452 are welded together and the arc plates 456are welded together in the circumferential direction of the body 101, toform a ring-shaped integrated structure. A resin 458 formed in an arcshape is placed in the space formed with the units 452, and in the spaceformed with the unit 452 and the arc plate 456, respectively.

[0161] In assembling of this cask 450, the resin 458 is first mounted onthe body 101, and the unit 452 a is covered to accommodate this resin458 in the inside. Then, the heat conductive fins 454 of the unit 452 aare mounted on bulge portions 459 provided around the body 101, and thewelding is carried out from both sides of the fins, which are open(welded joints 460). Then, the remaining latter half of the resin 458 ismounted on the body 101, and the unit 452 is covered to accommodate theresin 458 in the inside, in the same manner as explained above. At thesame time, the ends of the arc plates of the units are welded together.Further, the unit 452 b with the resin 458 b is welded in an axialdirection of the body 101 at a predetermined interval, and this processis repeated by a necessary number of times. When the unit 452 b is fixedto the body 101, the resin 458 c is disposed in advance between theunits 452 a and 452 b. This resin 458 c is engaged with the edges of thearc plate edges of the units 452 a and 452 b on both sides so as not tobe removed from this place.

[0162] The arc plates 456 are fitted in the whole gaps each between theunits 452, and the arc plates 453 and the arc plates 456 are welded fromthe outside. Then, the assembling of the external cylinder 451 isfinished. Further, instead of using a molded resin, it is also possibleto fill a resin from a hole provided on the arc plate 453 (not shown).Based on the above structure, it is possible to carry out all thewelding work from the outside, without welding in a narrow and longspace. Therefore, it is possible to carry out the welding easily, and aspecific exclusive welding machine is not necessary. Further, as thewelded joints 455 are separated from the welded joints 457, it ispossible to prevent local concentration of thermally influencedsections. Further, the welding may be carried out after such steps thatthe resin 458 (458 c) is divided into resins in the circumferentialdirection, the units 452 a and 452 b are welded respectively, thedivided resins 458 c are sequentially inserted into a space between theunits 452 a and 452 b, and the arc plate 456 is covered over the resin458 c (not shown).

[0163] Sixth Embodiment:

[0164]FIG. 26 is a perspective view which shows a cask according to thesixth embodiment of this invention. FIG. 27 is a cross-sectional view ofthe cask in its radial direction shown in FIG. 26. In this cask 1100, abody 1101 and a bottom plate 1104 of the body 1101 are forged productsthat are made of carbon steel having a gamma-ray shielding function. Itis possible to use stainless steel instead of carbon steel. The body1101 and the bottom plate 1104 are connected together by welding. Inorder to secure sealing performance of the cask as a pressure-resistantvessel, a metallic gasket is provided between a primary lid 1110 and thebody 1101 (not shown).

[0165] As shown in FIG. 28, sheet members 1201 are fixed to the externalperiphery of the body 1101. This sheet member 1201 is formed by bendinga rectangular iron plate, aluminum plate, or copper plate, and ischaracterized in that a section corresponding to the external cylinderof the cask 1100 is formed in an angular shape (an angular-shapedportion 1207). An edge 1202 of the sheet member 1201 is welded to a bentedge 1203 of an adjacent sheet member 1201 (welded joints 1202 a). Thesheet member 1201 functions as what is called a heat conductive fin thatreleases the decay heat of the spent fuel assemblies to the outside. Onthe other hand, the sheet member 1201 is fixed by passing a wire 1204through edge fixing holes 1205, and winding this wire 1204 around thebody 1101. The sheet member 1201 may be formed by extrusion.

[0166] A honeycomb member 1210 made of aluminum or copper havingexcellent thermal conductivity is provided in a space 1206 formedbetween the sheet member 1201 and the external surface of the body 1101.The cells of the honeycomb member 1210 are formed in an axial directionof the cask 1100, and the periphery of the cells is in contact with theinternal surface 1211 of the space 1206. This is because the efficiencyof conducting heat from the body 1101 is improved. A part of thishoneycomb member 1210 is filled with a resin 1106 that is a polymermaterial containing much hydrogen and having a neutron shieldingfunction. On the other hand, at the external peripheral side of thehoneycomb member 1210, a void layer 1212 not filled with the resin 1106is provided in order to absorb thermal expansion of the resin 1106 andthe like. The honeycomb member 1210 is filled with the resin 1106 to becombined together in some other place, and they are inserted into thespace 1206 and are fixed. Alternatively, the resin 1106 in a fluidstatus may be injected into the space 1206 in which the honeycomb member1210 is inserted, and cured based on a thermosetting reaction or thelike.

[0167] For the honeycomb member 1210, it is possible to selectively use,as required, any of various kinds of materials such as a roll core or acorrugated core in addition to an ordinary honeycomb member having ahexagonal shape in its cross section. FIG. 29 shows concrete examples.FIG. 29(a) shows an ordinary honeycomb member 1210 a having a hexagonalshape in its cross section. FIG. 29(b) shows a corrugated core typehoneycomb member 1210 b. FIG. 29(c) shows a honeycomb member 1210 chaving a laminated structure of members each of which is subjected todimple processing on one side thereof. A honeycomb member 1210 d shownin FIG. 29(d) has a structure of members each of which is subjected todimple processing on both sides thereof, and which are connected to eachother at projected front ends d1. A honeycomb member 1210 e shown inFIG. 29(e) has a structure of aluminum plates each of which is providedwith a plurality of holes e1 having returns e2, and which are connectedto each other at the return sections e2.

[0168] Other than the honeycomb member, it is also possible to use anymaterial that can contribute to the improvement of thermal conductivityby combining this material with the resin 1106. For example, as shown inFIG. 29(f), a Raschig Ring 1210 f as a heat conductor may be filled inthe space 206, and the resin 1106 may be filled around this Raschig Ring1210 f. Further, as shown in FIG. 29(g), a demister 1210 g made ofaluminum may be fitted in the space 1206, and the resin 1106 may befilled around this demister. From the viewpoint of thermal conductivity,it is preferable that the size of each cell is as small as possible.However, it is not desirable that this size is so small that it isdifficult to fill the resin 1106 in the cell.

[0169] The external surface shape of the sheet member 1210 may be otherthan these described above. For example, as shown in FIG. 30(a), theexternal surface of a sheet member 1220 may have a valley shape (avalley-shaped section 1221). Further, as shown in FIG. 30(b), twoangular-shaped portions 1231 may be provided on the external surface ofa sheet member 1230. Further, as shown in FIG. 30(c), the cross sectionof the external surface of a sheet member 1240 may have an arc shape (anarc section 1241). In addition to the shapes, it is also possible to usevarious kinds of angular-shaped portions or valley-shaped portions ifthe shapes have an increased heat radiation area.

[0170] It is preferable to form an obtuse angle θ at the angular-shapedportion 1207 or a valley portion 1208 that is formed on the externalsurface of the cask by providing the valley-shaped portion(corresponding to a joint of the sheet members when the angular-shapedportion is provided, and a valley portion when the valley-shaped portionis provided). With this arrangement, it becomes easy to carry outdecontamination of the cask 1100.

[0171] A lid section 1109 is constructed of a primary lid 1110 and asecondary lid 1111. This primary lid 1110 is a disk made of stainlesssteel or carbon steel that shields the gamma ray. The secondary lid 1111is also a disk made of stainless steel or carbon steel. A resin issealed in the upper surface of this secondary lid as a neutron shield(not shown). The primary lid 1110 and the secondary lid 1111 are fixedto a body 1101 with bolts made of stainless steel or carbon steel.Further, metallic gaskets are proved between the primary lid 1110 andthe secondary lid 1111 and the body 1101 respectively, thereby to holdair tightness in the inside. Trunnions 1117 are provided at both sidesof the cask body to suspend the cask 1100. Buffers 1118 are fixed toboth sides of the cask 1100 when the cask 1100 is transported.

[0172] The internal surface of the body 1101 has a shape that matchesthe external shape of a basket 1300 to be used. The external surface ofthe basket 1300 is in approximately close contact with the body 1101(however, a fine gap may be produced in actual cases). It is notnecessary to match the internal surface of the body 1101 with theexternal shape of the basket 1300 approximately completely. Instead, itis also possible to shape the internal surface such that a part of theexternal surface of the basket 1300 is not in contact with the internalsurface. It is possible to suitably design a proportion between the twoby taking thermal conductivity into account.

[0173] For the basket 1300, it is possible to use a structure of cellsformed by combining angular pipes made of aluminum composite material oraluminum alloy obtained by adding powder of B (boron) or a B compoundhaving a neutron absorbing function to powder of Al or an Al alloy. Itis also possible to use a laminated structure of forging having cells(corresponding to the basket shown in FIG. 26 and FIG. 27), or to use astructure laminated in a shape of a box for cakes by crossingrectangular plates of aluminum composite material or aluminum alloy withslits. For the neutron absorber, it is possible to use cadmium inaddition to boron. The basket 1300 according to this embodiment canaccommodate 33 spent fuel assemblies (for PWR), but the number is notlimited to 33. It is needless to mention that it is also possible to usethe basket as a cask for BWR.

[0174] As the external shape of the basket 1300 becomes close to anoctagonal shape, the internal surface of the body 1101 becomesapproximately octagonal. Therefore, the external shape of the body 1101is formed octagonal so as to face each surface. With this arrangement,the overall thickness of the body 1101 becomes approximately uniform,and there is no surplus thickness. Therefore, it is possible to reduceweight. Further, it is possible to secure the gamma-ray shieldingperformance within a necessarily sufficient range. The internal surfaceand the external surface of the body 1101 are machined using anexclusive machining unit. For details, refer to Japanese PatentApplication Laid-open No. 11-249314 filed by the applicant of thisapplication. While the shape of the body 1101 in this embodiment isoctagonal, the shape is not limited to this. In other words, it is alsopossible to set the shape of the body 1101 to a decagon or a dodecagonso as to match the external shape of the basket 1300 (not shown).

[0175] For accommodating spent fuel assemblies in the cask 1100, thecask 1100 is sunk in the pool, and the spent fuel assemblies areaccommodated under the water. Therefore, radioactive materials adhere tothe periphery of the cask 1100. Consequently, it is necessary to removethe radioactive materials when the cask is lifted up from the pool.However, as the conventional cask 1500 has a structure that thecomb-shaped fins 1504 are welded to the stripes 1502, a brush forremoving the radioactive materials is hard to reach a root part of thefins, and shaded sections occur when the cask is cleaned with sprayedwater. Therefore, there has been a problem of difficulty in the removalas it is necessary to clean thoroughly.

[0176] On the other hand, according to this embodiment, the cask asfollows has been proposed. That is, the cask comprises the body 1101internally provided with the basket 1300 where spent fuel assemblies areaccommodated, and the sheet members 1201 provided around the body 1101,where a part of each of the sheet members 1201 is in contact with thebody 1101, another part of the member constitutes an external surface ofthe cask 1100 and forms a single or plural angular-shaped portions 1207(or valley-shaped portions), and still another part of the member isconnected to the adjacent sheet member 1201, the resin 1106 is filled inthe space 1206 structured by the sheet members 1201, and the angleformed at the joint of the sheet members 1201 becomes the obtuse angleθ.

[0177] As explained above, by setting the angle at the joint to theobtuse angle θ, for example, about 120 degrees as shown in FIG. 27, abrush for decontamination can reach all corners of the external surfaceof the cask. Therefore, it becomes possible to carry out thedecontamination easily.

[0178] The spent fuel assemblies that are accommodated in the cask 1100contain fissionable materials and fission products, and generateradiation with decay heat. Therefore, it is necessary to securelymaintain the heat removing function, the shielding function, and thecritical-state preventing function of the cask 1100 during the storageperiod. According to this cask 1100, the sheet members 1201 are providedaround the body 1101, and the honeycomb member 1210 provided in thespace 1206 formed with the sheet members 1201 and the sheet members 1201that form the external surface of the cask 1100 are formed in angularshapes to expand a heat radiation area. With this arrangement, the decayheat produced from the spent fuel assemblies is conducted and radiatedefficiently.

[0179] The decay heat produced from the spent fuel assemblies isconducted to the body 1101 through the basket 1300 or the filled heliumgas. Next, the decay heat is conducted to the external surface of thecask 1100 through the sheet members 1201 having the function as internalfins and the honeycomb member 1210. Particularly, the honeycomb member1210 is made of aluminum or copper having excellent thermalconductivity, and therefore the honeycomb member efficiently absorbs theheat from the body 1101, conducts this heat to the sheet members 1201,and radiates the heat to the outside from the external surface of thecask. From the above, as the decay heat can be efficiently removed, itis possible to keep the temperature inside the cask to a lower levelthan a conventional example when the decay heat quantity is the same.

[0180] The gamma ray generated from the spent fuel assemblies isshielded by the body 1101, the sheet members 1201, and the lid section1109 that are made of carbon steel or stainless steel. Further, neutronis shielded by the resin 1106, thereby to avoid the influence ofexposure to radiation operators. Specifically, it is designed to obtaina shielding function such that a surface dose equivalent rate is 2 mSv/hor below, and a dose equivalent rate at 1 m deep from the surface is 100μSv/h or below. Further, the aluminum alloy containing boron is used forthe basket 1300, and therefore it is possible to prevent the spent fuelassemblies from reaching a critical state by absorbing neutrons.

[0181] According to the cask 1100 of this embodiment, the sheet members1201 are fixed to the body 1101, the angular-shaped portions 1207 areprovided at sections forming the external surface of the cask out of thesheet members 1201, and the aluminum-made honeycomb member 1210 isfilled into the space 1206 formed with the sheet members 1201.Therefore, it is possible to improve the efficiency of conducting thedecay heat. Further, according to the cask 1100 of this embodiment, itis not necessary to weld the plurality of fins 1504 onto the externalsurface like the conventional practice. Therefore, there is an advantagethat it does not require much labor in the manufacturing. While the cask1100 accommodates the spent fuel assemblies, it is needless to mentionthat the cask can accommodate other radioactive materials.

[0182]FIG. 31 shows a modification of this embodiment. According to thiscask, the sheet members 1201 made of carbon steel are provided aroundthe body 1101. A heat-conductive plate 1400 consisting of an aluminumplate (or a copper plate or a graphite sheet) is adhered to both sidesof the sheet member 1201. Only a resin 1106 is filled in the spaceformed with the sheet members 1201. An allowance for expansion 1401 isformed between the external side of the resin 1106 and the sheet member1201 (not shown). The aluminum plate 1400 may be provided on only asingle side of the sheet member 1201. Further, in order to increase theheat collection efficiency, the heat-conductive plate 1400 may beextended (an extension section 1402) and brought into surface contactwith the body 1101 as shown in FIG. 32. In this case, as heat isconducted efficiently from the extension section 1402 to theheat-conductive plate 1400, it is possible to keep the temperature ofthe resin 1106 at a low level. Therefore, it is possible to preventdeterioration in the quality of the resin 1106.

[0183] In the above structure, the heat-conductive plate 1400 (includingthe extension section 1402 of the heat-conductive plate in the case ofFIG. 32) conducts the main decay heat. Even if the honeycomb member 1210is used, the heat-conductive plate 1400 can be adhered to the sheetmember 1201. With this arrangement, it is possible to further improvethe thermal conductivity of the cask.

[0184] The shape of the heat-conductive plate 1400 is not particularlylimited. For example, as shown in FIG. 33, an extension section 1405maybe provided so as to be extended to the body 1101. According to thisstructure, the efficiency of collecting heat from the body 1101 isfurther improved, and it is possible to keep the temperature inside theresin 1106 and the body 1101 at a low level.

[0185] As shown in FIG. 34, it is also possible to provide anintermediate member 1406 that is a part of the sheet member 1201 made ofiron or the like having high heat resistance, in the section filled withthe resin 1106. A heat-conductive plate like an aluminum plate or acopper plate is used for the intermediate member 1406. Further, it ispreferable to use a material that can be easily welded to the body 1101,for a fitting section 1407 with the body 1101. The material includes,for example, the carbon steel or stainless steel that is the samematerial as that used for the body 1101. The sheet member 1201, theintermediate member 1406, and the fitting section 1407 are welded inadvance at some other place at a pre-stage of executing the welding tothe body 1101. Friction stirring welding or TIG welding is used for thewelding. FIG. 35 is a partially enlarged view of FIG. 34. The end of thefitting section 1407 is formed with an angled edge 1407 a for thewelding. With this arrangement, it is possible to increase the contactarea with the body 1101 according to a welding bulge portion 1407 b.Therefore, heat conduction improves.

[0186]FIG. 36 shows another modification. For a sheet member 1208, analuminum member having the function as a heat conductor that is the sameas the intermediate member 1406 is used. Further, a fitting section 1408with the body 1101 is provided at the end of this sheet member 1201. Forthe fitting section 1408, it is also preferable to use a material thatcan be easily welded to the body 1101. In this structure, a heatconductor like an aluminum material forms an external cylinder of thecask 1100. Therefore, heat movement is excellent. Based on the structureusing the aluminum material, it is possible to reduce the weight of thecask 1100. Such a structure has low requirement for heat resistance, andis therefore suitable for a container exclusive for storage.

[0187] The shape of the sheet member 1201 is not limited to the oneshown in FIG. 27. For example, as shown in FIG. 37, a sheet member 1250having a Y shape in its cross section may be used. This sheet member1250 can be formed by bending or extrusion. In the case of this sheetmember 1250, its leg is welded to the body 1101, and both edges areconnected to edges 1251 of adjacent sheet members 1250. Based on this,the sheet members 1250 form heat conductive fins and an external surfaceof the cask 1100. In this case, welded joints 1253 are positioned at aridgeline of an angular-shaped portion 1252. When the strength of thewelded joints 1253 becomes low, the welded joints 1253 may be positionedin the middle of the angular-shaped portion 1252 (not shown).

[0188] As shown in FIG. 38(a), a plurality of umbrella-shaped sheetmembers 1260 may be used to form a cask having valley-shaped portions1261. Further, as shown in FIG. 38(b), the external surface of a sheetmember 1270 having a T shape in its cross section is formed in a waveshape (a wave-shaped portion 271), and its edges 1272 are connected tothe edges of adjacent sheet members 1270. With this arrangement, it isalso possible to obtain the same effect as the above. These sheetmembers 1260 and 1270 may be formed by sheeting or may be molded byextrusion.

INDUSTRIAL APPLICABILITY

[0189] As explained above, according to the cask and the method ofmanufacturing the same of the present invention, it is possible toobtain the cask that is easily assembled and has high thermalconductivity. Therefore, the cask is suitable for accommodation of thespent fuel assemblies.

1. A cask comprising a body that accommodates a basket formed with aplurality of cells where spent fuel assemblies are accommodated, with aplurality of heat conductive fins provided around an external peripheryof the body, an external cylinder fixed to an external periphery of theheat conductive fins, and a neutron absorber provided in a space formedwith the heat conductive fins and the external cylinder, wherein theexternal cylinder has a structure that a plurality of belt-like membersA and B are welded along edges of the members, the heat conductive finsare welded along both sides of one belt-like member A with a margin leftalong edges of the both sides, and another belt-like member B adjacentto the belt-like member A is welded near the respective edges.
 2. A caskcomprising a body that accommodates a basket formed with a plurality ofcells where spent fuel assemblies are accommodated, with a plurality ofheat conductive fins provided around an external periphery of the body,an external cylinder fixed to an external periphery of the heatconductive fins, and a neutron absorber provided in a space formed withthe heat conductive fins and the external cylinder, wherein the externalcylinder has a structure that belt-like members A and B are welded alongedges of the members, a unit is structured by welding the heatconductive fins along both sides of one belt-like member A with a marginleft along edges of the both sides, the unit is welded to the body withthe heat conductive fins from the outside of the unit and is arranged atpredetermined intervals, and another belt-like member B is covered overa gap between the belt-like member A and the belt-like member A ofadjacent units and is welded from the outside.
 3. A cask comprising abody that accommodates a basket formed with a plurality of cells wherespent fuel assemblies are accommodated, with a plurality of heatconductive fins provided around an external periphery of the body, anexternal cylinder fixed to an external periphery of the heat conductivefins, and a neutron absorber provided in a space formed with the heatconductive fins and the external cylinder, wherein the external cylinderhas a structure that belt-like members A and B are welded along edges ofthe members, a unit is structured by welding the heat conductive finalong an approximately central part of one belt-like member A, the unitis welded to the body with the heat conductive fins and is arranged atpredetermined intervals, and another belt-like member B is covered overa gap between the belt-like member A and the belt-like member A ofadjacent units and is welded from the outside of the member B.
 4. A caskcomprising a body that accommodates a basket formed with a plurality ofcells where spent fuel assemblies are accommodated, with a plurality ofheat conductive fins provided around an external periphery of the body,an external cylinder fixed to an external periphery of the heatconductive fins, and a neutron absorber provided in a space formed withthe heat conductive fins and the external cylinder, wherein the externalcylinder has a structure that a plurality of belt-like members A arewelded along edges of the members, the heat conductive fin is weldedalong an approximately central part of the belt-like member A, the heatconductive fin is welded to the body from one side of the fin, andadjacent belt-like members A are welded near the edges of the members.5. The cask according to any one of claims 1 to 4, wherein the belt-likemember is formed in an angular shape or a valley shape by bending. 6.The cask according to claim 5, wherein the heat conductive fin is fixeddiagonally with respect to a radial direction of the cask.
 7. The caskaccording to any one of claims 1 to 4, wherein the heat conductive finis bent in an L shape or a U shape in its cross section, and is insurface contact with the belt-like member.
 8. A cask comprising a bodythat accommodates a basket formed with a plurality of cells where spentfuel assemblies are accommodated, with a plurality of heat conductivefins provided around an external periphery of the body, an externalcylinder fixed to an external periphery of the heat conductive fins, anda neutron absorber provided in a space formed with the heat conductivefins and the external cylinder, wherein the external cylinder has astructure that a belt-like member A having an angular shape or a valleyshape in its cross section is covered over external edges of adjacentheat conductive fins, and contacting edges of adjacent belt-like membersA are welded.
 9. The cask according to any one of claims 1 to 4 andclaim 8, wherein the adjacent belt-like members are coupled to eachother along ends of the members.
 10. A cask comprising a body thataccommodates a basket formed with a plurality of cells where spent fuelassemblies are accommodated, with a plurality of heat conductive finsprovided around an external periphery of the body, an external cylinderfixed to an external periphery of the heat conductive fins, and aneutron absorber provided in a space formed with the heat conductivefins and the external cylinder, wherein the external cylinder has astructure that a plurality of ring sheets are welded in an axialdirection of the cylinder, a unit is structured by welding a ring-shapedheat conductive fin along an approximately central part of an internalsurface of the ring sheet, the unit is welded to the body with the heatconductive fin from one side of the fin, and contacting edges ofadjacent ring sheets are welded from the outside of the ring sheet. 11.A cask comprising a body that accommodates a basket formed with aplurality of cells where spent fuel assemblies are accommodated, with aplurality of heat conductive fins provided around an external peripheryof the body, an external cylinder fixed to an external periphery of theheat conductive fins, and a neutron absorber provided in a space formedwith the heat conductive fins and the external cylinder, wherein theexternal cylinder has a structure that ring sheets A and B are welded inan axial direction of the cylinder, a unit is structured by weldingring-shaped heat conductive fins to both sides of one ring sheet A witha margin left along edges of the both sides, the unit is welded to thebody with the heat conductive fins from the outside of the unit and isarranged at predetermined intervals, and another ring sheet B isdisposed between the ring sheet A and the ring sheet A of adjacent unitsand is welded from the outside.
 12. The cask according to claim 11,wherein the unit and/or the ring sheet B is obtained by being dividedinto parts in a circumferential direction of the cask and welding theseparts together.
 13. The cask according to any one of claims 10 to 12,wherein the neutron absorber provided in the space formed with the heatconductive fins and the external cylinder is a molded resin formed alongthe shape of the space.
 14. The cask according to any one of claims 1 to4, 8, 10, and 11, wherein for welding the heat conductive fins to thebody made of steel, a bulge portion made of iron is provided on thebody, and the heat conductive fin made of copper is welded to the bulgeportion.
 15. A cask comprising a heat conductive fin and an externalcylinder that are divided into a plurality of units, each unit beingconnected around an external periphery of a body of the cask, and thenadjacent external cylinders being connected together so as to enableheat conduction.
 16. A cask comprising an external cylinder formed witha plurality of units that are disposed around an external periphery of abody of the cask, each of the units being integrally formed with a heatconductive fin and a wall of a part of the external cylinder in advance,and the heat conductive fins, the body, and adjacent walls of theexternal cylinder being connected together to enable heat conduction.17. A cask comprising: a body having a cavity that accommodates fuelassemblies; a basket formed with sheet members that partition inside thecavity in a lattice shape; heat conductive plates provided on endsurfaces of the sheet member that are in contact with the cavity; and aplurality of units each of which is integrally formed with a heatconductive f in connected to an external periphery of the body and anexternal cylinder covering over the external periphery of the body. 18.A method of manufacturing a cask, the method comprising: a step offorming a unit by welding the heat conductive fins along both sides of abelt-like member A with a margin left along edges of the both sides; astep of welding a plurality of the units to a body of the cask with theheat conductive fins from the outside of each of the units; and a stepof fitting a belt-like member B into a gap between the belt-like memberA and an adjacent belt-like member A, and welding along contacting edgesof the belt-like members A and B from the outside.
 19. A method ofmanufacturing a cask, the method comprising: a step of forming a unit bywelding a heat conductive f in along an approximately central part of abelt-like member A; a step of welding at least two units to a body ofthe cask with the heat conductive fin by arranging the units atpredetermined intervals; and a step of covering another belt-like memberB over a gap between the belt-like member A and the belt-like member Aof adjacent units and welding the belt-like member B from the outside.20. A method of manufacturing a cask, the method comprising: a step ofwelding a heat conductive fin along an approximately central part of abelt-like member A; a step of welding the belt-like member A to a bodyof the cask with the heat conductive fin from an open side of the fin;and a step of welding a next belt-like member A to the body with a heatconductive fin from an open side of the fin, and welding adjacentbelt-like members A together along contacting edges of the members A.21. A method of manufacturing a cask, the method comprising: a step offorming a unit by welding a ring-shaped heat conductive fin along anapproximately central part of the internal surface of a ring sheet A; astep of welding the unit to a body of the cask with the heat conductivefin in an axial direction of the cask from an open side of the fin; anda step of welding a next unit to the body with the heat conductive finin the axial direction from an open side of the fin, and weldingcontacting edges of adjacent ring sheets A together from the outside.22. A method of manufacturing a cask, the method comprising: a step offorming a unit by welding a ring-shaped heat conductive fin along anapproximately central part of an internal surface of a ring sheet A; astep of welding the unit to a body of the cask with the heat conductivefin in an axial direction of the cask from an open side of the fin; astep of accommodating approximately a half of a molded resin formed in aring shape into the unit; and a step of welding a next unit to the bodywith a heat conductive fin in the axial direction from an open side ofthe fin, welding contacting edges of adjacent ring sheets A togetherfrom the outside, and accommodating a remaining half of the formedmolded resin in the unit.
 23. A method of manufacturing a cask, themethod comprising: a step of forming a unit by welding ring-shaped heatconductive fins along both sides of one ring sheet A with a margin leftalong edges of the both sides; a step of welding at least two units tothe body with the heat conductive fins in an axial direction of the caskfrom the outside of the units by arranging the units at predeterminedintervals; and a step of disposing another ring sheet B in a gap betweenthe ring sheet A and the ring sheet A of adjacent units and welding thering sheet B from the outside.
 24. A method of manufacturing a cask, themethod comprising: a step of forming a unit by welding ring-shaped heatconductive fins along both sides of one ring sheet A with a margin leftalong edges of the both sides; a step of accommodating a ring-shapedresin into the unit; a step of welding at least two units to a body ofthe cask with the heat conductive fins in an axial direction of the caskfrom the outside of the units by arranging the units at predeterminedintervals; a step of disposing a resin molded in a ring-shape intobetween adjacent units; and a step of covering another ring sheet B overa gap between the ring sheet A and the ring sheet A of adjacent unitsand welding the ring sheet B from the outside.
 25. The method ofmanufacturing a cask according to claim 20 or 21, wherein the unitand/or the ring sheet B has a structure obtained by being divided intoparts in a circumferential direction of the cask, and these dividedparts are individually welded to the body.
 26. A cask comprising a bodyinternally provided with a basket that accommodates radioactivematerials such as spent fuel assemblies, and a plurality of sheetmembers provided around the body, wherein a part of each of the membersis in contact with the body, another part of the member constitutes anexternal surface of the cask and forms a single or plural angular-shapedportions or valley-shaped portions, and still another part of the memberis connected to an adjacent sheet member, and a neutron absorber isfilled in a space formed with the sheet members.
 27. The cask accordingto claim 26, wherein the neutron absorber and a honeycomb member arecombined, and the honeycomb member is in contact at least with the body.28. The cask according to claim 26, wherein the neutron absorber iscombined with a plurality of heat conductors.
 29. The cask according toclaim 27 or 28, wherein the honeycomb member or the heat conductor ismade of aluminum or copper, and the sheet member is made of iron,aluminum, or copper.
 30. The cask according to claim 29, wherein analuminum plate, copper plate, or a graphite sheet is adhered onto asingle side or both sides of the sheet member.
 31. The cask according toany one of claims 26 to 28, wherein a part of the sheet membercontacting the body is in surface contact with the body.
 32. The caskaccording to claim 30, wherein a part of the aluminum plate or thecopper plate is in surface contact with the body.
 33. A cask comprising:a body internally provided with a basket that accommodates radioactivematerials such as spent fuel assemblies; a plurality of sheet membersprovided around the body, with a part of each of the members being incontact with the body, another part of the member constituting anexternal surface of the cask, and still another part of the member beingconnected to an adjacent sheet member; and a heat good conductor that isadhered to the sheet member and has an extension section that reaches tocontact the body.
 34. A cask comprising: a body internally provided witha basket that accommodates radioactive materials such as spent fuelassemblies; and a plurality of sheet members provided around the body,with a part of each of the members being in contact with the body,another part of the member constituting an external surface of the cask,and still another part of the member being connected to an adjacentsheet member, the sheet member further including a fitting section withthe body, and an intermediate member connected to the fitting section,wherein the fitting section has higher weldability with the body thanother portions of the sheet member, and the intermediate member hashigher thermal conductivity than other portions of the sheet member. 35.A cask comprising: a body internally provided with a basket thataccommodates radioactive materials such as spent fuel assemblies; and aplurality of sheet members provided around the body, with a part of eachof the members being in contact with the body, another part of themember constituting an external surface of the cask, and still anotherpart of the member being connected to an adjacent sheet member, whereinthe sheet member includes a portion that functions as a heat conductivefin and a portion that functions as an external cylinder, the portionsbeing made of different materials from each other, and thermalconductivity of the portion that functions as the heat conductive fin isrelatively high, and heat resistance of the portion that functions asthe external cylinder is relatively high.
 36. A cask comprising: a bodyinternally provided with a basket that accommodates radioactivematerials such as spent fuel assemblies; and a plurality of sheetmembers provided around the body, with a part of each of the membersbeing in contact with the body, another part of the member constitutingan external surface of the cask, and still another part of the memberbeing connected to an adjacent sheet member, wherein the sheet memberfurther includes a fitting section with the body, and the fittingsection has higher weldability with respect to the body than otherportions of the sheet member.
 37. A cask comprising: a basket that isprovided with cells where spent fuel assemblies are accommodated and hasan approximately octagonal external shape, a body that accommodates thebasket, has its external shape and internal shape formed to match thebasket, and shields a gamma ray, and a neutron shield externallyprovided around the body.
 38. The cask according to claim 37, whereinthe neutron shield is provided in a polygonal shape relative to thebody.
 42. (New) The cask according to claim 39, wherein the heatconductive fin is bent in an L shape or a U shape in its cross section,and is in surface contact with the belt-like member.
 43. (New) The caskaccording to claim 39, wherein the adjacent belt-like members arecoupled to each other along ends of the members.
 44. (New) The caskaccording to claim 39, wherein for welding the heat conductive fins tothe body made of steel, a bulge portion made of iron is provided on thebody, and the heat conductive fin made of copper is welded to the bulgeportion.
 45. (New) A cask comprising a body that accommodates a basketformed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder, wherein the external cylinder has a structure thatfirst and second belt-like members are welded along edges of themembers, a unit is structured by welding the heat conductive fins alongboth sides of one first belt-like member with a margin left along edgesof the both sides, the unit is welded to the body with the heatconductive fins from the outside of the unit and is arranged atpredetermined intervals, and a second belt-like member is covered over agap between the first belt-like members of adjacent units and is weldedfrom the outside.
 46. (New) The cask according to claim 45, wherein thebelt-like member is formed in an angular shape or a valley shape bybending.
 47. (New) The cask according to claim 46, wherein the heatconductive fin is fixed diagonally with respect to a radial direction ofthe cask.
 48. (New) The cask according to claim 45, wherein the heatconductive fin is bent in an L shape or a U shape in its cross section,and is in surface contact with the belt-like member.
 49. (New) The caskaccording to claim 45, wherein the adjacent belt-like members arecoupled to each other along ends of the members.
 50. (New) The caskaccording to claim 47, wherein for welding the heat conductive fins tothe body made of steel, a bulge portion made of iron is provided on thebody, and the heat conductive fin made of copper is welded to the bulgeportion.
 51. (New) A cask comprising a body that accommodates a basketformed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder, wherein the external cylinder has a structure thatfirst and second belt-like members are welded along edges of themembers, a unit is structured by welding the heat conductive fin alongan approximately central part of one first belt-like member, the unit iswelded to the body with the heat conductive fins and is arranged atpredetermined intervals, and a second belt-like member is covered over agap between the first belt-like members of adjacent units and is weldedfrom the outside.
 52. (New) The cask according to claim 51, wherein thebelt-like member is formed in an angular shape or a valley shape bybending.
 53. (New) The cask according to claim 52, wherein the heatconductive fin is fixed diagonally with respect to a radial direction ofthe cask.
 54. (New) The cask according to claim 51, wherein the heatconductive fin is bent in an L shape or a U shape in its cross section,and is in surface contact with the belt-like member.
 55. (New) The caskaccording to claim 51, wherein the adjacent belt-like members arecoupled to each other along ends of the members.
 56. (New) The caskaccording to claim 51, wherein for welding the heat conductive fins tothe body made of steel, a bulge portion made of iron is provided on thebody, and the heat conductive fin made of copper is welded to the bulgeportion.
 57. (New) A cask comprising a body that accommodates a basketformed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder, wherein the external cylinder has a structure that aplurality of belt-like members A are welded along edges of the members,the heat conductive fin is welded along an approximately central part ofthe first belt-like member, the heat conductive fin is welded to thebody from one side of the fin, and adjacent belt-like members A arewelded near the edges of the members.
 58. (New) The cask according toclaim 57, wherein the belt-like member is formed in an angular shape ora valley shape by bending.
 59. (New) The cask according to claim 58,wherein the heat conductive fin is fixed diagonally with respect to aradial direction of the cask.
 60. (New) The cask according to claim 57,wherein the heat conductive fin is bent in an L shape or a U shape inits cross section, and is in surface contact with the belt-like member.61. (New) The cask according to claim 57, wherein the adjacent belt-likemembers are coupled to each other along ends of the members.
 62. (New)The cask according to claim 57, wherein for welding the heat conductivefins to the body made of steel, a bulge portion made of iron is providedon the body, and the heat conductive fin made of copper is welded to thebulge portion.
 63. (New) A cask comprising a body that accommodates abasket formed with a plurality of cells where spent fuel assemblies areaccommodated, with a plurality of heat conductive fins provided aroundan external periphery of the body, an external cylinder fixed to anexternal periphery of the heat conductive fins, and a neutron absorberprovided in a space formed with the heat conductive fins and theexternal cylinder, wherein the external cylinder has a structure that afirst belt-like member having an angular shape or a valley shape in itscross section is covered over external edges of adjacent heat conductivefins, and contacting edges of adjacent first belt-like members arewelded.
 64. (New) The cask according to claim 63, wherein the adjacentbelt-like members are coupled to each other along ends of the members.65. (New) The cask according to claim 63, wherein for welding the heatconductive fins to the body made of steel, a bulge portion made of ironis provided on the body, and the heat conductive fin made of copper iswelded to the bulge portion.
 66. (New) A cask comprising a body thataccommodates a basket formed with a plurality of cells where spent fuelassemblies are accommodated, with a plurality of heat conductive finsprovided around an external periphery of the body, an external cylinderfixed to an external periphery of the heat conductive fins, and aneutron absorber provided in a space formed with the heat conductivefins and the external cylinder, wherein the external cylinder has astructure that a plurality of ring sheets are welded in an axialdirection of the cylinder, a unit is structured by welding a ring-shapedheat conductive fin along an approximately central part of an internalsurface of the ring sheet, the unit is welded to the body with the heatconductive fin from one side of the fin, and contacting edges ofadjacent ring sheets are welded from the outside of the ring sheet. 67.(New) The cask according to claim 66, wherein the neutron absorberprovided in the space formed with the heat conductive fins and theexternal cylinder is a molded resin formed along the shape of the space.68. (New) The cask according to claim 66, wherein for welding the heatconductive fins to the body made of steel, a bulge portion made of ironis provided on the body, and the heat conductive fin made of copper iswelded to the bulge portion.
 69. (New) A cask comprising a body thataccommodates a basket formed with a plurality of cells where spent fuelassemblies are accommodated, with a plurality of heat conductive finsprovided around an external periphery of the body, an external cylinderfixed to an external periphery of the heat conductive fins, and aneutron absorber provided in a space formed with the heat conductivefins and the external cylinder, wherein the external cylinder has astructure that first and second ring sheets are welded in an axialdirection of the cylinder, a unit is structured by welding ring-shapedheat conductive fins to both sides of one first ring sheet with a marginleft along edges of the both sides, the unit is welded to the body withthe heat conductive fins from the outside of the unit and is arranged atpredetermined intervals, and a second ring sheet is disposed between thefirst ring sheets of adjacent units and is welded from the outside. 70.(New) The cask according to claim 69, wherein any or both of the unitand the second ring sheet is obtained by being divided into parts in acircumferential direction of the cask and these parts are weldedtogether.
 71. (New) The cask according to claim 69, wherein the neutronabsorber provided in the space formed with the heat conductive fins andthe external cylinder is a molded resin formed along the shape of thespace.
 72. (New) The cask according to claim 69, wherein for welding theheat conductive fins to the body made of steel, a bulge portion made ofiron is provided on the body, and the heat conductive fin made of copperis welded to the bulge portion.
 73. (New) A cask comprising a heatconductive fin and an external cylinder that are divided into aplurality of units, each unit being connected around an externalperiphery of a body of the cask, and then adjacent external cylindersbeing connected together so as to enable heat conduction.
 74. (New) Acask comprising an external cylinder formed with a plurality of unitsthat are disposed around an external periphery of a body of the cask,each of the units being integrally formed with a heat conductive fin anda wall of a part of the external cylinder in advance, and the heatconductive fins, the body, and adjacent walls of the external cylinderbeing connected together to enable heat conduction.
 75. (New) A caskcomprising: a body having a cavity that accommodates fuel assemblies; abasket formed with sheet members that partition inside the cavity in alattice shape; heat conductive plates provided on end surfaces of thesheet member that are in contact with the cavity; and a plurality ofunits each of which is integrally formed with a heat conductive finconnected to an external periphery of the body and an external cylindercovering over the external periphery of the body.
 76. (New) A method ofmanufacturing a cask, the method comprising: a step of forming a unit bywelding heat conductive fins along both sides of a first belt-likemember with a margin left along edges of the both sides; a step ofwelding a plurality of the units to a body of the cask with the heatconductive fins from the outside of each of the units; and a step offitting a second belt-like member into a gap between the first belt-likemember and an adjacent first belt-like member, and welding alongcontacting edges of the first and second belt-like members from theoutside.
 77. (New) A method of manufacturing a cask, the methodcomprising: a step of forming a unit by welding a heat conductive finalong an approximately central part of a first belt-like member; a stepof welding at least two units to a body of the cask with the heatconductive fin by arranging the units at predetermined intervals; and astep of covering a second belt-like member over a gap between the firstbelt-like members of adjacent units and welding the second belt-likemember from the outside.
 78. (New) A method of manufacturing a cask, themethod comprising: a step of welding a heat conductive fin along anapproximately central part of a first belt-like member; a step ofwelding the first belt-like member to a body of the cask with the heatconductive fin from an open side of the fin; and a step of welding anext first belt-like member to the body with a heat conductive fin froman open side of the fin, and welding adjacent belt-like members Atogether along contacting edges of the members A.
 79. (New) The methodof manufacturing a cask according to claim 78, wherein any or both ofthe unit and the second ring sheet has a structure obtained by beingdivided into parts in a circumferential direction of the cask, and thesedivided parts are individually welded to the body.
 80. (New) A method ofmanufacturing a cask, the method comprising: a step of forming a unit bywelding a ring-shaped heat conductive fin along an approximately centralpart of the internal surface of a first ring sheet; a step of weldingthe unit to a body of the cask with the heat conductive fin in an axialdirection of the cask from an open side of the fin; and a step ofwelding a next unit to the body with the heat conductive fin in theaxial direction from an open side of the fin, and welding contactingedges of adjacent first ring sheets together from the outside.
 81. (New)The method of manufacturing a cask according to claim 80, wherein any orboth of the unit and the second ring sheet has a structure obtained bybeing divided into parts in a circumferential direction of the cask, andthese divided parts are individually welded to the body.
 82. (New) Amethod of manufacturing a cask, the method comprising: a step of forminga unit by welding a ring-shaped heat conductive fin along anapproximately central part of an internal surface of a first ring sheet;a step of welding the unit to a body of the cask with the heatconductive fin in an axial direction of the cask from an open side ofthe fin; a step of accommodating approximately a half of a molded resinformed in a ring shape into the unit; and a step of welding a next unitto the body with a heat conductive fin in the axial direction from anopen side of the fin, welding contacting edges of adjacent ring sheets Atogether from the outside, and accommodating a remaining half of theformed molded resin in the unit.
 83. (New) A method of manufacturing acask, the method comprising: a step of forming a unit by weldingring-shaped heat conductive fins along both sides of one first ringsheet with a margin left along edges of the both sides; a step ofwelding at least two units to the body with the heat conductive fins inan axial direction of the cask from the outside of the units byarranging the units at predetermined intervals; and a step of disposinga second ring sheet in a gap between the first ring sheets of adjacentunits and welding the second ring sheet from the outside.
 84. (New) Amethod of manufacturing a cask, the method comprising: a step of forminga unit by welding ring-shaped heat conductive fins along both sides ofone first ring sheet with a margin left along edges of the both sides; astep of accommodating a ring-shaped resin into the unit; a step ofwelding at least two units to a body of the cask with the heatconductive fins in an axial direction of the cask from the outside ofthe units by arranging the units at predetermined intervals; a step ofdisposing a resin molded in a ring-shape into between adjacent units;and a step of covering a second ring sheet over a gap between the firstring sheets of adjacent units and welding the second ring sheet from theoutside.
 85. (New) A cask comprising a body internally provided with abasket that accommodates radioactive materials such as spent fuelassemblies, and a plurality of sheet members provided around the body,wherein a part of each of the members is in contact with the body,another part of the member constitutes an external surface of the caskand forms a single or plural angular-shaped portions or valley-shapedportions, and still another part of the member is connected to anadjacent sheet member, and a neutron absorber is filled in a spaceformed with the sheet members.
 86. (New) The cask according to claim 85,wherein the neutron absorber and a honeycomb member are combined, andthe honeycomb member is in contact at least with the body.
 87. (New) Thecask according to claim 85, wherein the neutron absorber is combinedwith a plurality of heat conductors.
 88. (New) The cask according toclaim 86, wherein the honeycomb member or the heat conductor is made ofaluminum or copper, and the sheet member is made of iron, aluminum, orcopper.
 89. (New) The cask according to claim 88, wherein an aluminumplate, copper plate, or a graphite sheet is adhered onto a single sideor both sides of the sheet member.
 90. (New) The cask according to claim85, wherein a part of the sheet member contacting the body is in surfacecontact with the body.
 91. (New) The cask according to claim 89, whereina part of the aluminum plate or the copper plate is in surface contactwith the body.
 92. (New) A cask comprising: a body internally providedwith a basket that accommodates radioactive materials such as spent fuelassemblies; a plurality of sheet members provided around the body, witha part of each of the members being in contact with the body, anotherpart of the member constituting an external surface of the cask, andstill another part of the member being connected to an adjacent sheetmember; and a heat good conductor that is adhered to the sheet memberand has an extension section that reaches to contact the body.
 93. (New)A cask comprising: a body internally provided with a basket thataccommodates radioactive materials such as spent fuel assemblies; and aplurality of sheet members provided around the body, with a part of eachof the members being in contact with the body, another part of themember constituting an external surface of the cask, and still anotherpart of the member being connected to an adjacent sheet member, thesheet member further including a fitting section with the body, and anintermediate member connected to the fitting section, wherein thefitting section has higher weldability with the body than other portionsof the sheet member, and the intermediate member has higher thermalconductivity than other portions of the sheet member.
 94. (New) A caskcomprising: a body internally provided with a basket that accommodatesradioactive materials such as spent fuel assemblies; and a plurality ofsheet members provided around the body, with a part of each of themembers being in contact with the body, another part of the memberconstituting an external surface of the cask, and still another part ofthe member being connected to an adjacent sheet member, wherein thesheet member includes a portion that functions as a heat conductive finand a portion that functions as an external cylinder, the portions beingmade of different materials from each other, and thermal conductivity ofthe portion that functions as the heat conductive fin is relativelyhigh, and heat resistance of the portion that functions as the externalcylinder is relatively high.
 95. (New) A cask comprising: a bodyinternally provided with a basket that accommodates radioactivematerials such as spent fuel assemblies; and a plurality of sheetmembers provided around the body, with a part of each of the membersbeing in contact with the body, another part of the member constitutingan external surface of the cask, and still another part of the memberbeing connected to an adjacent sheet member, wherein the sheet memberfurther includes a fitting section with the body, and the fittingsection has higher weldability with respect to the body than otherportions of the sheet member.